Engineering Use of Geo Textiles

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					                                                UFC 3-220-08FA
                                                16 January 2004




UNIFIED FACILITIES CRITERIA (UFC)



        ENGINEERING USE OF
           GEOTEXTILES




   APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED
                                                                            UFC 3-220-08FA
                                                                            16 January 2004



                          UNIFIED FACILITIES CRITERIA (UFC)

                         ENGINEERING USE OF GEOTEXTILES

Any copyrighted material included in this UFC is identified at its point of use.
Use of the copyrighted material apart from this UFC must have the permission of the
copyright holder.



U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)

NAVAL FACILITIES ENGINEERING COMMAND

AIR FORCE CIVIL ENGINEER SUPPORT AGENCY



Record of Changes (changes are indicated by \1\ ... /1/)

Change No.      Date             Location




This UFC supersedes TM 5-818-8, dated 20 July 1995. The format of this UFC does not conform to
UFC 1-300-01; however, the format will be adjusted to conform at the next revision. The body of
this UFC is the previous TM 5-818-8, dated 20 July 1995.




                                               1
                                                                              UFC 3-220-08FA
                                                                              16 January 2004
                                         FOREWORD
\1\
The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and provides
planning, design, construction, sustainment, restoration, and modernization criteria, and applies
to the Military Departments, the Defense Agencies, and the DoD Field Activities in accordance
with USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and
work for other customers where appropriate. All construction outside of the United States is
also governed by Status of forces Agreements (SOFA), Host Nation Funded Construction
Agreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.)
Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, the
SOFA, the HNFA, and the BIA, as applicable.

UFC are living documents and will be periodically reviewed, updated, and made available to
users as part of the Services’ responsibility for providing technical criteria for military
construction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval Facilities
Engineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) are
responsible for administration of the UFC system. Defense agencies should contact the
preparing service for document interpretation and improvements. Technical content of UFC is
the responsibility of the cognizant DoD working group. Recommended changes with supporting
rationale should be sent to the respective service proponent office by the following electronic
form: Criteria Change Request (CCR). The form is also accessible from the Internet sites listed
below.

UFC are effective upon issuance and are distributed only in electronic media from the following
source:

•   Whole Building Design Guide web site http://dod.wbdg.org/.

Hard copies of UFC printed from electronic media should be checked against the current
electronic version prior to use to ensure that they are current.

AUTHORIZED BY:


______________________________________              ______________________________________
DONALD L. BASHAM, P.E.                              DR. JAMES W WRIGHT, P.E.
Chief, Engineering and Construction                 Chief Engineer
U.S. Army Corps of Engineers                        Naval Facilities Engineering Command


______________________________________              ______________________________________
KATHLEEN I. FERGUSON, P.E.                          Dr. GET W. MOY, P.E.
The Deputy Civil Engineer                           Director, Installations Requirements and
DCS/Installations & Logistics                          Management
Department of the Air Force                         Office of the Deputy Under Secretary of Defense
                                                       (Installations and Environment)




                                                2
                                                ARMY TM 5-818-8
                                       AIR FORCE AFJMAN 32-1030
                       TECHNICAL MANUAL




ENGINEERING USE OF GEOTEXTILES




            APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED



DEPARTMENTS OF THE ARMY AND THE AIR FORCE
                                   20 July 1995
                                              TM 5-818-8/AFJMAN 32-l030




       REPRODUCTION AUTHORIZATION/RESTRICTIONS

This manual has been prepared by or for the Government and,
except to the extent indicated below, is public property and not
subject to copyright.
Reprints or republications of this manual should include a credit
substantially as follows: “Joint Departments of the Army and Air
Force, TM 5-818-8/AFJMAN 32-1030, Engineering Use of Geotex-
tiles,” 20 July 1995.
                                                                                                                                                            TM 5-818-8/AFJMAN 32-l030

TECHNICAL MANUAL                                                                                                                                              HEADQUARTERS
No. 5-818-8                                                                                                                                         DEPARTMENTS OF THE ARMY
AIR FORCE MANUAL                                                                                                                                           AND THE AIR FORCE
No. 32-1030                                                                                                                                          WASHINGTON, DC, 20 July 1995
                                                   ENGINEERING USE OF GEOTEXTILES
                                                                                                                                                               Paragraph     Page
CHAPTER 1 . INTRODUCTION
             Purpose.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              1-1         1-1
             Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          1-2         1-1
             References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             1-3         1-1
             Geotextile Types and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    1-4         1-1
             Geotextile Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        1-5         1-2
             Seam Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    1-6         1-2
             Geotextile Functions and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                        1-7         1-3
         2. GEOTEXTILES IN PAVEMENT APPLICATIONS
             Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               2-1         2-1
             Paved Surface Rehabilitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                2-2         2-1
             Reflective Crack Treatment for Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                             2-3         2-1
             Separation and Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                 2-4         2-2
             Design for Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        2-5         24
             Geotextile Survivability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          2-6         24
             Design for Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                             2-7         2-4
         3. FILTRATION AND DRAINAGE
             Water Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  3-1         3-1
             Granular Drain Performance.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                   3-2         3-1
             Geotextile Characteristics Influencing Filter Functions                                                                                              3-3         3-1
             Piping Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    34          3-1
             Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 3-5         3-2
             Other Filter Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                              3-6         3-3
             Strength Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                           3-7         3-4
             Design and Construction Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                          3-8         3-4
          4. GEOTEXTILE REINFORCED EMBANKMENT ON SOFT FOUNDATION
             General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            4-1         4-1
             Potential Embankment Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           4-2         4-1
             Recommended Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                           4-3         4-3
             Example Geotextile-Reinforced Embankment Design. . . . . . . . . . . . . . . . . . . . . . .                                                         44          4-7
             Bearing-Capacity Consideration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                  4-5         4-8
          5. RAILROAD TRACK CONSTRUCTION AND REHABILITATION
             General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            5-1         5-1
             Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     5-2         5-1
             Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              5-3         5-1
             Depth of Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       54          5-1
             Protective Sand Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          5-5         5-2
             Drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             5-6         5-3
             Typical Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   5-7         5-3
             Special Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       5-8         5-3
          6. EROSION AND SEDIMENT CONTROL
             Erosion Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  6-1         6-1
             Bank Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 6-2         6-1
             Precipitation Runoff Collection and Diversion Ditches. . . . . . . . . . . . . . . . . . . . . .                                                     6-3         6-3
             Miscellaneous Erosion Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                 64          6-3
             Sediment Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     6-5         6-4
         7 . REINFORCED SOIL WALLS
             Geotextile-Reinforced Soil Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                 7-1         7-1
             Advantages of Geotextile-Reinforced Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                            7-2         7-1
             Disadvantages of Geotextile- Reinforced Walls. . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                               7-3         7-1
             Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         74          7-1
             General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                           7-5         7-1
             Properties of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        7-6         7-2
             Design Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    7-7         7-3
             Design Procedure......................................................................                                                               7-8         7-3




                                           APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED

                                                                                                                                                                                    i
TM 5-818-8/AFJMAN 32-l030

                                                                                                                                            Paragraph             Page
APPENDIX    A. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   A-l
BIBLIOGRAPHY..........................................................................................BIBLIOGRAPHY-l

                                                                              LIST OF FIGURES
Figure     1-1 Dimensions and Directions for Woven Geotextiles.                                                                                                   1-2
           1-2. Woven Monofilament Geotextiles Having Low Percent Open Area (Top), and High Percent Open                                                          1-3
                 Area (Bottom).
           1-3. Woven Multifilament Geotextile.                                                                                                                   1-4
           1-4. Woven Slit-Film Geotextile.                                                                                                                       1-4
           1-5. Needle-Punched Nonwoven Geotextile.                                                                                                               1-5
           1-6. Heat-Bonded Nonwoven Geotextile.                                                                                                                  1-6
           1-7. Seam Types Used in Field Seaming of Geotextiles.                                                                                                  1-7
           1-8. Stitch Types Used in Field Seaming of Geotextiles.                                                                                                1-8
           2-1. Geotextile in AC Overlay.                                                                                                                         2-2
           2-2. Guidance for Geotextile Use in Minimizing Reflective Cracking.                                                                                    2-3
           2-3. Relationship Between Shear Strength, CBR, and Cone Index.                                                                                         2-6
           2-4. Thickness Design Curve for Single-Wheel Load on Gravel-Surfaced Roads.                                                                            2-7
           2-5. Thickness Design Curve for Dual-Wheel Load on Gravel- Surfaced Roads.                                                                             2-8
           2-6. Thickness Design Curve for Tandem-Wheel Load on Gravel-Surfaced Roads.                                                                            2-9
           3-1. Trench Drain Construction.                                                                                                                        3-5
           4-1. Potential Geotextile-Reinforced Embankment Failure Modes.                                                                                         4-2
           4-2. Concept Used for Determining Geotextile Tensile Strength Necessary to Prevent Slope Failure.                                                      4-4
           4-3. Assumed Stresses and Strains Related to Lateral Earth Pressures.                                                                                  4-7
           4-4. Embankment Section and Foundation Conditions of Embankment Design Example Problem.
           5-1. Typical Sections of Railroad Track with Geotextile.                                                                                               5-4
           6-1. Relationship between Atterberg Limits and Expected Erosion Potential.                                                                             6-2
           6-2. Pin Spacing Requirements in Erosion Control Applications.                                                                                         6-3
           6-3. Geotextile Placement for Currents Acting Parallel to Bank or for Wave Attack on the Bank.                                                         6-4
           6-4. Ditch Liners.                                                                                                                                     6-5
           6-5. Use of Geotextiles near Small Hydraulic Structures.                                                                                               6-6
           6-6. Use of Geotextiles around Piers and Abutments.                                                                                                    6-6
           6-7. Sedimentation behind Silt Fence.                                                                                                                  6-7
           7-1. General Configuration of a Geotextile Retained Soil Wall and Typical Pressure Diagrams.                                                           7-2
           7-2. Procedures for Computing Live Load Stresses on Geotextile Reinforced Retaining Walls.                                                             7-4

                                                                                LIST OF TABLES
Table      2-1. Property Requirements of Nonwoven Geotextiles.                                                                                                     2-3
           2-2. Construction Survivability Ratings (FHWA 1989).                                                                                                    2-4
           2-3. Relationship of Construction Elements to Severity of Loading Imposed on Geotextile in Road-                                                        2-5
                 way Construction (FHWA 1989).
           2-4. Minimum Geotextile Strength Properties for Survivability (FHWA 1989).                                                                              2-5
           3-1. Geotextile Filter Design Criteria.                                                                                                                 3-1
           3-2. Geotextile Strength Requirements for Drains.                                                                                                       3-4
           5-1. Recommended Geotextile Property Requirements for Railroad Applications.                                                                            5-2
           6-1. Recommended Geotextile Minimum Strength Requirements.                                                                                              6-2
           6-2. Pin Spacing Requirements in Erosion Control Applications.                                                                                          6-3




ii
                                                                                       TM 5-818-8/AFJMAN 32-l030

                                                        CHAPTER 1

                                                    INTRODUCTION

1-1. Purpose                                                     as shown in fig 1-1). Woven construction produces
This manual describes various geotextiles, test                  geotextiles with high strengths and moduli in the
methods for evaluating their properties, and rec-                warp and fill directions and low elongations at
ommended design and installation procedures.                     rupture. The modulus varies depending on the rate
                                                                 and the direction in which the geotextile is loaded.
1-2. Scope                                                       When woven geotextiles are pulled on a bias, the
This manual covers physical properties, functions,               modulus decreases, although the ultimate break-
design methods, design details and construction                  ing strength may increase. The construction can
procedures for geotextiles as used in pavements,                 be varied so that the finished geotextile has equal
railroad beds, retaining wall earth embankment,                  or different strengths in the warp and fill direc-
rip-rap, concrete revetment, and drain construc-                 tions. Woven construction produces geotextiles
tion. Geotextile functions described include pave-               with a simple pore structure and narrow range of
ments, filtration and drainage, reinforced embank-               pore sizes or openings between fibers. Woven
ments, railroads, erosion and sediment control,                  geotextiles are commonly plain woven, but are
and earth retaining walls. This manual does not                  sometimes made by twill weave or leno weave (a
cover the use of other geosynthetics such as geo-                very open type of weave). Woven geotextiles can be
grids, geonets, geomembranes, plastic strip drains,              composed of monofilaments (fig l-2) or multifila-
composite products and products made from natu-                  ment yarns (fig 1-3). Multifilament woven con-
ral cellulose fibers.                                            struction produces the highest strength and modu-
                                                                 lus of all the constructions but are also the highest
1-3. References                                                  cost. A monofilament variant is the slit-film or
Appendix A contains a list of references used in                 ribbon filament woven geotextile (fig l-4). The
this manual.                                                     fibers are thin and flat and made by cutting sheets
                                                                 of plastic into narrow strips. This type of woven
1-4. Geotextile Types and Construction                           geotextile is relatively inexpensive and is used for
   a. Materials. Geotextiles are made from poly-                 separation, i.e., the prevention of intermixing of
propylene, polyester, polyethylene, polyamide                    two materials such as aggregate and fine-grained
(nylon), polyvinylidene chloride, and fiberglass.                soil.
Polypropylene and polyester are the most used.                         (2) Manufacturers literature and textbooks
Sewing thread for geotextiles is made from                       should be consulted for greater description of
Kevlar L or any of the above polymers. The physi-                woven and knitted geotextile manufacturing pro-
cal properties of these materials can be varied by               cesses which continue to be expanded.
the use of additives in the composition and by                         (3) Nonwoven geotextiles are formed by a
changing the processing methods used to form the                 process other than weaving or knitting, and they
molten material into filaments. Yarns are formed                 are generally thicker than woven products. These
from fibers which have been bundled and twisted                  geotextiles may be made either from continuous
together, a process also referred to as spinning.                filaments or from staple fibers. The fibers are
(This reference is different from the term spinning              generally oriented randomly within the plane of
as used to denote the process of extruding fila-                 the geotextile but can be given preferential orien-
ments from a molten material.) Yarns may be                      tation. In the spunbonding process, filaments are
composed of very long fibers (filaments) or rela-                extruded, and laid directly on a moving belt to
tively short pieces cut from filaments (staple                   form the mat, which is then bonded by one of the
fibers).                                                         processes described below.
   b. Geotextile Manufacture.
                                                                         (a) Needle punching. Bonding by needle
     (1) In woven construction, the warp yarns,
                                                                 punching involves pushing many barbed needles
which run parallel with the length of the geotex-
                                                                 through one or several layers of a fiber mat
tile panel (machine direction), are interlaced with
                                                                 normal to the plane of the geotextile. The process
yarns called fill or filling yarns, which run perpen-
                                                                 causes the fibers to be mechanically entangled (fig
dicular to the length of the panel (cross direction
                                                                 l-5). The resulting geotextile has the appearance
1                                                                of a felt mat.
  Kevlar is a registered trademark of Du Pont for their aramid
fiber.                                                                   (b) Heat bonding. This is done by incorpo-

                                                                                                                  1-1
TM 5-818-8/AFJMAN 32-1030




                              Figure 1-1. Dimensions and Directions for Woven Geotextiles.
rating fibers of the same polymer type but having              material cannot easily be periodically inspected or
different melting points in the mat, or by using               easily replaced if it should become degraded (for
heterofilaments, that is, fibers composed of one               example filtration and/or drainage functions
type of polymer on the inside and covered or                   within an earth dam), current practice is to use
sheathed with a polymer having a lower melting                 only geologic materials (which are orders of magni-
point. A heat-bonded geotextile is shown in figure             tude more resistant to these weathering effects
l-6.                                                           than polyesters).
      (c) Resin bonding. Resin is introduced into
the fiber mat, coating the fibers and bonding the              1-6. Seam Strength
contacts between fibers.                                          a. Joining Panels. Geotextile sections can be
       (d) Combination bonding. Sometimes a com-               joined by sewing, stapling, heat welding, tying,
bination of bonding techniques is used to facilitate           and gluing. Simple overlapping and staking or
manufacturing or obtain desired properties.                    nailing to the underlying soil may be all that is
     (4) Composite geotextiles are materials which             necessary where the primary purpose is to hold
combine two or more of the fabrication techniques.             the material in place during installation. However,
The most common composite geotextile is a non-                 where two sections are joined and must withstand
woven mat that has been bonded by needle punch-                tensile stress or where the security of the connec-
ing to one or both sides of a woven scrim.                     tion is of prime importance, sewing is the most
                                                               reliable joining method.
1-5. Geotextile Durability                                        b. Sewn Seams. More secure seams can be pro-
Exposure to sunlight degrades the physical proper-             duced in a manufacturing plant than in the field.
ties of polymers. The rate of degradation is re-               The types of sewn seams which can be produced in
duced by the addition of carbon black but not                  the field by portable sewing machines are pre-
eliminated. Hot asphalt can approach the melting               sented in figure 1-7. The seam type designations
point of some polymers. Polymer materials become               are from Federal Standard 751. The SSa seam is
brittle in very cold temperatures. Chemicals in the            referred to as a “prayer” seam, the SSn seam as a
groundwater can react with polymers. All poly-                  “J” seam, and the SSd as a “butterfly” seam. The
mers gain water with time if water is present.                 double-sewn seam, SSa-2, is the preferred method
High pH water can be harsh on polyesters while                 for salvageable geotextiles. However, where the
low pH water can be harsh on polyamides. Where                 edges of the geotextile are subject to unraveling,
a chemically unusual environment exists, labora-               SSd or SSn seams are preferred.
tory test data on effects of exposure of the geotex-              c. Stitch Type. The portable sewing machines
tile to this environment should be sought. Experi-             used for field sewing of geotextiles were designed
ence with geotextiles in place spans only about 30             as bag closing machines. These machines can
years. All of these factors should be considered in            produce either the single-thread or two-thread
selecting or specifying acceptable geotextile mate-            chain stitches as shown in figure l-8. Both of
rials. Where long duration integrity of the mate-              these stitches are subject to unraveling, but the
rial is critical to life safety and where the in-place         single-thread stitch is much more susceptible and

1-2
                                                                                       TM 5-818-8/AFJMAN 32-1030




    Figure 1-2. Woven Monofilament Geotextiles Having Low Percent Open Area (Top), and High Percent Open Area (Bottom)
must be tied at the end of each stitching. Two                  though it may be desirable to permit the thread to
rows of stitches are preferred for field seaming,               be made of a material different from the geotextile
and two rows of stitches are absolutely essential               being sewn. Sewing thread for geotextiles is usu-
for secure seams when using the type 101 stitch                 ally made from Kevlar, polyester, polypropylene,
since, with this stitch, skipped stitches lead to               or nylon with the first two recommended despite
complete unraveling of the seam. Field sewing                   their greater expense. Where strong seams are
should be conducted so all stitching is exposed for             required, Kevlar sewing thread provides very high
inspection. Any skipped stitches should be over-                strength with relative ease of sewing.
sewn.
   d. Sewing Thread. The composition of the                     1-7 Geotextile Functions and Applications.
thread should meet the same compositional perfor-                 a. Functions. Geotextiles perform one or more
mance requirements as the geotextile itself, al-                basic functions: filtration, drainage, separation,

                                                                                                                         1-3
TM 5-818-8/AFJMAN 32-1030




                                  Figure l-3. Woven Multifilament Geotextile.




                                    Figure 1-4. Woven Slit-Film Geotextile.
erosion control, sediment control, reinforcement,           b. Filtration. The use of geotextiles in filter
and (when impregnated with asphalt) moisture              applications is probably the oldest, the most
barrier. In any one application, a geotextile may         widely known, and the most used function of
be performing several of these functions.                 geotextiles. In this application, the geotextile is


1-4
                                                                                    TM 5-818-8/AFJMAN 32-1030




                                  Figure l-5. Needle-Punched Nonwoven Geotextile.
placed in contact with and down gradient of soil to         to long term clogging potential of geotextile
be drained. The plane of the geotextile is normal           drains. They are known to be effective in short
to the expected direction of water flow. The capac-         duration applications.
ity for flow of water normal to the plane of the               d. Erosion Control. In erosion control, the geo-
geotextile is referred to as permittivity. Water and        textile protects soil surfaces from the tractive
any particles suspended in the water which are              forces of moving water or wind and rainfall ero-
smaller than a given size flow through the geotex-          sion. Geotextiles can be used in ditch linings to
tile. Those soil particles larger than that size are        protect erodible fine sands or cohesionless silts.
stopped and prevented from being carried away.              The geotextile is placed in the ditch and is secured
The geotextile openings should be sized to prevent          in place by stakes or is covered with rock or gravel
soil particle movement. The geotextiles substitute          to secure the geotextile, shield it from ultraviolet
for and serve the same function as the traditional          light, and dissipate the energy of the flowing
granular filter. Both the granular filter and the
                                                            water. Geotextiles are also used for temporary
geotextile filter must allow water (or gas) to pass
                                                            protection against erosion on newly seeded slopes.
without significant buildup of hydrostatic pres-
                                                            After the slope has been seeded, the geotextile is
sure. A geotextile-lined drainage trench along the
                                                            anchored to the slope holding the soil and seed
edge of a road pavement is an example using a
                                                            in-place until the seeds germinate and vegetative
geotextile as a filter. Most geotextiles are capable
                                                            cover is established. The erosion control function
of performing this function. Slit film geotextiles
are not preferred because opening sizes are unpre-          can be thought of as a special case of the combina-
dictable. Long term clogging is a concern when              tion of the filtration and separation functions.
geotextiles are used for filtration.                           e. Sediment Control. A geotextile serves to con-
   c. Drainage. When functioning as a drain, a              trol sediment when it stops particles suspended in
geotextile acts as a conduit for the movement of            surface fluid flow while allowing the fluid to pass
liquids or gases in the plane of the geotextile.            through. After some period of time, particles accu-
Examples are geotextiles used as wick drains and            mulate against the geotextile, reducing the flow of
blanket drains. The relatively thick nonwoven               fluid and increasing the pressure against the
geotextiles are the products most commonly used.            geotextile. Examples of this application are silt
Selection should be based on transmissivity, which          fences placed to reduce the amount of sediment
is the capacity for in-plane flow. Questions exist as       carried off construction sites and into nearby

                                                                                                            1-5
TM 5-818-8/AFJMAN 32-1030

water courses. The sediment control function is               ballast to prevent contamination and resulting
actually a filtration function.                               strength loss of the ballast by intrusion of the
  f. Reinforcement. In the most common reinforce-             subgrade soil. In construction of roads over soft
ment application, the geotextile interacts with soil          soil, a geotextile can be placed over the soft
through frictional or adhesion forces to resist               subgrade, and then gravel or crushed stone placed
tensile or shear forces. To provide reinforcement, a          on the geotextile. The geotextile prevents mixing
geotextile must have sufficient strength and em-              of the two materials.
bedment length to resist the tensile forces gener-
                                                                h. Moisture Barrier. Both woven and nonwoven
ated, and the strength must be developed at
sufficiently small strains (i.e. high modulus) to             geotextiles can serve as moisture barriers when
prevent excessive movement of the reinforced                  impregnated with bituminous, rubber-bitumen, or
structure. To reinforce embankments and retain-               polymeric mixtures. Such impregnation reduces
ing structures, a woven geotextile is recommended             both the cross-plane and in-plane flow capacity of
because it can provide high strength at small                 the geotextiles to a minimum. This function plays
strains.                                                      an important role in the use of geotextiles in
  g. Separation. Separation is the process of pre-            paving overlay systems. In such systems, the
venting two dissimilar materials from mixing. In              impregnated material seals the existing pavement
this function, a geotextile is most often required to         and reduces the amount of surface water entering
prevent the undesirable mixing of fill and natural            the base and subgrade. This prevents a reduction
soils or two different types of fills. A geotextile can       in strength of these components and improves the
be placed between a railroad subgrade and track               performance of the pavement system.




                                     Figure 1-6. Heat-Bonded Nonwoven Geotextile.




1-6
                                                            TM 5-818-8/AFJMAN 32-1030




SSa-1                                               SSa-2

                      PRAYER SEAM




SSd-1                                                SSd-2



                    BUTTERFLY SEAM




                                                     SSn-2

                          J SEAM
  Figure l-7. Seam Types Used in Field Seaming of Geotextiles.




                                                                                  1-7
TM 5-818-8/AFJMAN 32-1030



                      DIRECTION OF SUCCESSIVE STITCH FORMATION




                     STITCH TYPE 101. ONE-THREAD CHAIN STITCH




                       DIRECTION OF SUCCESSIVE STITCH FORMATION




                     STITCH TYPE 401, TWO-THREAD CHAIN STITCH

                            Figure 1-8. Stitch Types Used in Field Seaming of Geotextiles.




1-8
                                                                            TM 5-818-8/AFJMAN 32-1030

                                               CHAPTER 2
                         GEOTEXTILES IN PAVEMENT APPLICATIONS

2-1.   Applications                                    2-3. Reflective Crack Treatment for Pave-
This chapter discusses the use of geotextiles for      ments
asphalt concrete (AC) overlays on roads and air-          a. General. Geotextiles can be used successfully
fields and the separation and reinforcement of         in pavement rehabilitation projects. Conditions
materials in new construction. The functions per-      that are compatible for the pavement applications
formed by the geotextile and the design consider-      of geotextiles are AC pavements that may have
ations are different for these two applications. In    transverse and longitudinal cracks but are rela-
an AC pavement system, the geotextile provides a       tively smooth and structurally sound, and PCC
stress-relieving interlayer between the existing       pavements that have minimum slab movement.
pavement and the overlay that reduces and re-          The geographic location and climate of the project
tards reflective cracks under certain conditions       site have an important part in determining
and acts as a moisture barrier to prevent surface      whether or not geotextiles can be successfully used
water from entering the pavement structure.            in pavement rehabilitation. Geotextiles have been
When a geotextile is used as a separator, it is        successful in reducing and retarding reflective
placed between the soft subgrade and the granular      cracking in mild and dry climates when tempera-
                                                       ture and moisture changes are less likely to
material. It acts as a filter to allow water but not
                                                       contribute to movement of the underlying pave-
fine material to pass through it, preventing any
                                                       ment; whereas, geotextiles in cold climates have
mixing of the soft soil and granular material
                                                       not been as successful. Figure 2-2 gives guidance
under the action of the construction equipment or
                                                       in using geotextiles to minimize reflective crack-
subsequent traffic.                                    ing on AC pavements. Geotextiles interlayers are
                                                       recommended for use in Areas I and II, but are not
2-2. Paved Surface Rehabilitation
                                                       recommended for use in Area III. Since geotextiles
   a. General. Old and weathered pavements con-        do not seem to increase the performance of thin
tain transverse and longitudinal cracks that are       overlays, minimum overlay thicknesses for Areas I
both temperature and load related. The method          and II are given in figure 2-2. Even when the
most often used to rehabilitate these pavements is     climate and thickness requirements are met, there
to overlay the pavement with AC. This tempo-           has been no consistent increase in the time it
rarily covers the cracks. After the overlay has        takes for reflective cracking to develop in the
been placed, any lateral or vertical movement of       overlay indicating that other factors are influenc-
the pavement at the cracks due to load or ther-        ing performance. Other factors affecting perfor-
mal effects causes the cracks from the existing        mance of geotextile interlayers are construction
pavement to propagate up through the new AC            techniques involving pavement preparation, as-
overlay (called reflective cracking). This movement    phalt sealant application, geotextile installation,
causes raveling and spalling along the reflective      and AC overlay as well as the condition of the
cracks and provides a path for surface water to        underlying pavement.
                                                          b. Surface Preparation. Prior to using geotex-
reach the base and subgrade which decreases the
                                                       tiles to minimize reflective cracks, the existing
ride quality and accelerates pavement deteriora-
                                                       pavement should be evaluated to determine pave-
tion.
                                                       ment distress. The size of the cracks and joints in
   b. Concept. Under an AC overlay, a geotextile
                                                       the existing pavement should be determined. All
may provide sufficient tensile strength to relieve     cracks and joints larger than ¼ inch in width
stresses exerted by movement of the existing           should be sealed. Differential slab movement
pavement. The geotextile acts as a stress-relieving    should be evaluated, since deflections greater than
interlayer as the cracks move horizontally or          0.002 inch cause early reflective cracks. Areas of
vertically. A typical pavement structure with a        the pavement that are structurally deficient
geotextile interlayer is shown in figure 2-1. Im-      should be repaired prior to geotextile installation.
pregnation of the geotextile with a bitumen pro-       Placement of a leveling course is recommended
vides a degree of moisture protection for the          when the existing pavement is excessively cracked
underlying layers whether or not reflective crack-     and uneven.
ing occurs.                                               c. Geotextile Selection.
                                                                                                       2-1
TM 5-818-8/AFJMAN 32-1030




                                               BASE         COURSE




                                                    SUBGRADE
                                       Figure 2-1. Geotextile in AC Overlay.

     (1) Geotextile interlayers are used in two dif-        ever is greater. For AC pavements, Area I shown
ferent capacities-the full-width and strip methods.         in figure 2-2 should have a minimum overlay
The full-width method involves sealing cracks and           thickness of 2 inches; whereas, Area II should
joints and placing a nonwoven material across the           have a minimum overlay thickness of 3 inches.
entire width of the existing pavement. The mate-            The minimum thickness of an AC overlay for
rial should have the properties shown in table 2-1.         geotextile application on PCC pavements is 4
Nonwoven materials provide more flexibility and             inches.
are recommended for reflective crack treatment of             f. Spot Repairs. Rehabilitation of localized dis-
AC pavements.                                               tressed areas and utility cuts can be improved
     (2) The strip method is primarily used on PCC          with the application of geotextiles. Isolated dis-
pavements and involves preparing the existing               tressed areas that are excessively cracked can be
cracks and joints, and placing a 12 to 24 inch wide         repaired with geotextiles prior to an AC overlay.
geotextile and sufficient asphalt directly on the           Either a full-width membrane strip application can
cracks and joints. The required physical properties         be used depending on the size of the distressed
 are shown in table 2-1, however nonwoven geotex-           area. Localized distressed areas of existing AC
tiles are not normally used in the strip method.            pavement that are caused by base failure should
Membrane systems have been developed for strip              be repaired prior to any pavement rehabilitation.
repairs.                                                    Geotextiles are not capable of bridging structur-
   d. Asphalt Sealant. The asphalt sealant is used          ally deficient pavements.
to impregnate and seal the geotextile and bond it
                                                             2-4. Separation and Reinforcement
to both the base pavement and overlay. The grade
 of asphalt cement specified for hot-mix AC pave-            Soft subgrade materials may mix with the granu-
 ments in each geographic location is generally the          lar base or subbase material as a result of loads
 most acceptable material. Either anionic or catio-          applied to the base course during construction
 nic emulsion can also be used. Cutback asphalts             and/or loads applied to the pavement surface that
 and emulsions which contain solvents should not             force the granular material downward into the soft
 be used.                                                    subgrade or as a result of water moving upward
   e. AC Overlay. The thickness of the AC overlay            into the granular material and carrying the sub-
 should be determined from the pavement struc-               grade material with it. A sand blanket or filter
 tural requirements outlined in TM 5-822-5/                  layer between the soft subgrade and the granular
 AFJMAN 32-1018, TM 5-825-2/AFJMAN                           material can be used in this situation. Also, the
 32-1014 and TM 5-825-3/AFJMAN 32-1014,                      subgrade can be stabilized with lime or cement or
 Chap. 3 or from minimum requirements, which-                the thickness of granular material can be in-
2-2
                                                                                                     TM 5-818-8/AFJMAN 32-1030




                  AREA I- INTERLAYERS ARE RECOMMENDED WITH MINIMUM
                          OVERLAY THICKNESS OF 2 IN.
                 AREA II- INTERLAYERS ARE RECOMMENDED WITH OVERLAY
                         THICKNESS OF 3-4 IN.
                AREA III -INTERLAYERS ARE NOT RECOMMENDED.
                               Figure 2-2. Guidance for Geotextile Use in Minimizing Reflective Cracking.


                                       Table 2-1. Property Requirements of Nonwoven Geotextiles.

Property                                                            Requirements                     Test Method
Breaking load, pounds/inch of width                                 80 minimum                       ASTM D 4632
Elongation-at-break, percent                                        50 minimum                       ASTM D 4632
Asphalt retention, gallons per square yard                          0.2 minimum                      AASHTO M288
Melting point, degrees Fahrenheit                                   300 minimum                      ASTM D 276
Weight, ounce per square yard                                       3-9                              ASTM D 3776 Option B

creased to reduce the stress on the subgrade.                          separator to prevent the mixing of the soft soil and
Geotextiles have been used in construction o f                         the granular material, and (3) a reinforcement
gravel roads and airfields over soft soils to solve                    layer to resist the development of rutting. The
these problems and either increase the life of the                     reinforcement application is primarily for gravel
pavement or reduce the initial cost. The placement                     surfaced pavements. The required thicknesses of
of a permeable geotextile between the soft sub-                        gravel surfaced roads and airfields have been
grade and the granular material may provide one                        reduced because of the presence of the geotextile.
or more of the following functions, (1) a filter to                    There is no established criteria for designing
allow water but not soil to pass through it, (2) a                     gravel surfaced airfields containing a geotextile.


                                                                                                                            2-3
TM 5-818-8/AFJMAN 32-1030

2-5. Design for Separation                                     conditions and construction equipment; whereas,
                                                               table 2-3 relates survivability to cover material
When serving as a separator, the geotextile pre-
                                                               and construction equipment. Table 2-4 gives mini-
vents fines from migrating into the base course
                                                               mum geotextile grab, puncture, burst, and tear
and/or prevents base course aggregate from pene-
                                                               strengths for the survivability required for the
trating into, the subgrade. The soil retention prop-
                                                               conditions indicated in tables 2-2 and 2-3.
erties of the geotextile are basically the same as
those required for drainage or filtration. Therefore,          2-7. Design for Reinforcement
the retention and permeability criteria required               Use of geotextiles for reinforcement of gravel
for drainage should be met. In addition, the geo-              surfaced roads is generally limited to use over soft
textile should withstand the stresses resulting                cohesive soils (CBR < 4). One procedure for
from the load applied to the pavement. The nature              determining the thickness requirements of aggre-
of these stresses depend on the condition of the               gate above the geotextile was developed by the US
subgrade, type of construction equipment, and the              Forest Service (Steward, et al. 1977) and is as
cover over the subgrade. Since the geotextile                  follows:
serves to prevent aggregate from penetrating the                  a. Determine In-Situ Soil Strength. Determine
subgrade, it must meet puncture, burst, grab and               the in-situ soil strength using the field California
tear strengths specified in the following para-                Bearing Ratio (CBR), cone penetrometer, or Vane
graphs.                                                        Shear device. Make several readings and use the
                                                               lower quartile value.
2-6.    Geotextile   Survivability                                b. Convert Soil Strength. Convert the soil
Table 2-2 has been developed for the Federal                   strength to an equivalent cohesion (C) value using
Highway Administration (FHWA) to consider sur-                 the correlation shown in figure 2-3. The shear
vivability requirements as related to subgrade                 strength is equal to the C value.

                                Table 2-2. Construction Survivability Ratings (FHWA 1989)



       Site Soil CBR                          <1                             1-2                   >2
       at Installation
       Equipment Ground                >50           <50             >50           <50       >50         <50
       Contact Pressure
       (psi)
       Cover Thickness
           1
       (in.) (Compacted)
       4 2,3                             NR            NR              H             M         M           M
       6                                 NR            NR              H             H         M           M
       12                                NR            H               M             M         M           M
       18                                H             M               M             M         M           M

       H = High, M = Medium, NR = Not recommended.
       'Maximum aggregate size not to exceed one half the compacted cover
       thickness.
       2
       3
        For low volume unpaved road (ADT 200 vehicles).
         The four inch minimum cover is limited to existing road bases and
       not intended for use in new construction.




2-4
                                                                                         TM 5-818-8/AFJMAN 32-1030

Table 2-3. Relationship of Construction Elements to Severity of Loading Imposed on Geotextile in Roadway Construction.


                                                       Severity Category
Variable                      LOW                        Moderate        High to Very High
Equipment            Light weight                 Medium weight                     Heavy weight dozer;
                     dozer (8 psi)                dozer; light                      loaded dump truck
                                                  wheeled equipment                 (>40 psi)
                                                  (8-40 psi)
Subgrade             Cleared                      Partially cleared                 Not cleared
Condition
Subgrade             <0.5                         1-2                               >3
Strength
(CBR)
Aggregate            Rounded sandy                Coarse angular                    Cobbles, blasted
                     gravel                       gravel                            rock
Lift                 18                           12                                 6
Thickness
(in.)




                          Table 24. Minimum Geotextile Strength Properties for Survivability.

       Required
        Degree                                                Puncture                 Burst               Trap
      of Geotextile              Grab    Strength'            Strength'              Strength 3            Tear 4
      Survivability                     lb                       lb                     psi                 1b
     Very high                          270                        110                   430                75
     High                               180                         75                   290                50

     Moderate                           130                         40                   210                40

     Low                                  90                        30                   145                30




     Note:     All values represent minimum average roll values (i.e., any roll in a
               lot should meet or exceed the minimum values in this table). These
               values are normally 20 percent lower than manufacturers reported
               typical values.

     'ASTM D 4632.

     'ASTM D 4833.
     3
         ASTM D 3786.
     4
         ASTM D 4533, either principal direction.




                                                                                                                         2-5
TM 5-818-8lAFJMAN 32-1030

                                                                 (2) For thickness design using geotextile.
                                                                   (a) A value of 5.0 for NC would result in a
                                                           thickness design that would perform with very
                                                           little rutting (less than 2 inches) at traffic vol-
                                                           umes greater than 1,000 equivalent 18-kip axle
                                                           loadings.
                                                                   (b) A value of 6.0 for NC would result in a
                                                           thickness design that would rut 4 inches or more
                                                           under a small amount of traffic (probably less than
                                                           100 equivalent 18-kip axle loadings).
                                                              e. Geotextile reinforced gravel road design exam-
                                                           ple. Design a geotextile reinforced gravel road for
                                                           a 24,000-pound-tandem-wheel load on a soil having
                                                           a CBR of 1. The road will have to support several
                                                           thousand truck passes and very little rutting will
                                                           be allowed.
                                                                 (1) Determine the required aggregate thick-
                                                           ness with geotextile reinforcement.
                                                                    (a) From figure 2-3 a 1 CBR is equal to a C
                                                           value of 4.20.
   Figure 2-3. Relationship Between Shear Strength, CBR,            (b) Choose a value of 5 for NC since very
                       and Cone Index.                     little rutting will be allowed.
                                                                    (c) Calculate CN C as: CN C = 4.20(5) = 21.
   c. Select Design Loading. Select the desired de-                 (d) Enter figure 2-6 with CN C of 21 to
sign loading, normally the maximum axle loads.             obtain a value of 14 inches as the required
   d. Determine Required Thickness of Aggregate.           aggregate thickness above the geotextile.
Determine the required thickness of aggregate                       (e) Select geotextile requirements based on
above the geotextile using figures 2-4, 2-5, and           survivability requirements in tables 2-2 and 2-3.
2-6. These figures relate the depth of aggregate                 (2) Determine the required aggregate thick-
above the geotextile to the cohesion of the soil (C)       ness when a geotextile is not used.
and to a bearing capacity factor (NC). The product                  (a) Use a value of 2.8 for NC since a geotex-
of C and NC is the bearing capacity for a rapidly          tile is not used and only a small amount of rutting
loaded soil without permitting drainage. The sig-          will be allowed.
nificance of the value used for NC as it relates to                 (b) C a l c u l a t e C NC as: CN C = 4.20(2.8) =
the design thickness using figures 2-4, 2-5, and            11.8.
2-6 is as follows:                                                  (c) Enter figure 2-6 with CN C of 11.8 to
      (1) For thickness design without using geotex-       obtain a value of 22 inches as the required
tile.                                                      aggregate thickness above the subgrade without
        (a) A value of 2.8 for NC would result in a        the geotextile.
thickness design that would perform with very                    (3) Compare cost and benefits of the alterna-
little rutting (less than 2 inches) at traffic volumes     tives. Even with nearby economical gravel sources,
greater than 1,000 equivalent 18-kip axle loadings.        the use of a geotextile usually is the more econom-
        (b) A value of 3.3 for NC would result in a         ical alternative for constructing low volume roads
thickness design that would rut 4 inches or more            and airfields over soft cohesive soils. Additionally,
under a small amount of traffic (probably less than         it results in a faster time to completion once the
100 equivalent 18-kip axle loadings).                       geotextiles are delivered on site.




2-6
                                                                       TM 5-818-8/AFJMAN 32-1030




Figure 2-4. Thickness Design Curve for Single- Wheel Load on Gravel-Surfaced Roads.




                                                                                          2-7
TM 5-818-8/AFJMAN 32-1030




               Figure 2-5. Thickness Design Curve for Dual- Wheel Load on Gravel-Surfaced Roads.




2-8
                                                                         TM 5-818-8/AFJMAN 32-1030




Figure 2-6. Thickness Design Curve for Tandem- Wheel Load on Gravel-Surfaced Roads.




                                                                                             2-9
                                                                                   TM 5-818-8/AFJMAN 32-1030

                                               CHAPTER 3

                                   FILTRATION AND DRAINAGE

3-1 Water Control                                       retention), flow capacity, and clogging potential.
Control of water is critical to the performance of      These properties are indirectly measured by the
buildings, pavements, embankments, retaining            apparent opening size (AOS) (ASTM D 4751),
                                                        permittivity (ASTM D 4491), and gradient ratio
walls, and other structures. Drains are used to
relieve hydrostatic pressure against underground        test (ASTM D 5101). The geotextile must also have
and retaining walls, slabs, and underground tanks       the strength and durability to survive construction
                                                        and long-term conditions for the design life of the
and to prevent loss of soil strength and stability in
slopes, embankments, and beneath pavements. A           drain. Additionally, construction methods have a
properly functioning drain must retain the sur-         critical influence on geotextile drain performance.
rounding soil while readily accepting water from        3-4. Piping Resistance
the soil and removing it from the area. These             a. Basic Criteria. Piping resistance is the ability
general requirements apply to granular and geo-         of a geotextile to retain solid particles and is
textile filters. While granular drains have a long      related to the sizes and complexity of the openings
performance history, geotextile use in drains is        or pores in the geotextile. For both woven and
relatively recent and performance data are limited      nonwoven geotextiles, the critical parameter is the
to approximately 25 years. Where not exposed to         AOS. Table 3-1 gives the relation of AOS to the
sunlight or abrasive contact with rocks moving in       gradation of the soil passing the number 200 sieve
response to moving surface loads or wave action,        for use in selecting geotextiles.
long-term performance of properly selected geotex-
tiles has been good. Since long-term experience is                  Table 3-1. Geotextile Filter Design Criteria.
limited, geotextiles should not be used as a substi-
tute for granular filters within or on the upstream     Protected Soil                                    Permeability
                                                        (Percent Passing
face of earth dams or within any inaccessible           No. 200 Sieve)       Piping 1                 Woven Nonwoven 2
portion of the dam embankment. Geotextiles have         Less than 5%         AOS (mm) <0.6          POA 3 > 10% k G > 5k S
been used in toe drains of embankments where                                   (mm)
they are easily accessible if maintenance is re-                              (Greater than #30
quired and where malfunction can be detected.                                 US Standard
                                                                                 Sieve)
Caution is advised in using geotextiles to wrap         5 to 50%             AOS (mm) < 0.6     POA > 4% k G > 5k S
permanent piezometers and relief wells where they                             (mm)
form part of the safety system of a water retaining                           (Greater than #30
structure. Geotextiles have been used to prevent                              US Standard
infiltration of fine-grained materials into piezo-                                Sieve)
                                                        50 to 85%            AOS (mm) < 0.297 POA > 4% kG > 5kS
meter screens but long-term performance has not                                (mm)
been measured.                                                             (Greater than #50
                                                                           US Standard
3-2. Granular Drain Performance                                              Sieve)
To assure proper performance in granular drains,         Greater than 85% AOS (mm) < 0.297                          kG > 5k S
                                                                           (mm)
the designer requires drain materials to meet                              (Greater than #50
grain-size requirements based on grain size of the                         US Standard
surrounding soil. The two principal granular filter                          Sieve)
criteria, piping and permeability, have been devel-
                                                        1
oped empirically through project experience and            When the protected soil contains appreciable quantities of
                                                          material retained on the No. 4 sieve use only the soil passing
laboratory testing. The piping and permeability
                                                          the No. 4 sieve in selecting the AOS of the geotextile.
criteria are contained in TF 5-820-2/ AFJMAN            2
                                                           k, is the permeability of the nonwoven geotextile and k S is
32-1016, Chap. 2.                                         the permeability of the protected soil.
                                                        3
                                                           POA = Percent Open Area.
3-3. Geotextile Characteristics Influencing Fil-
                                                            b. Percent Open Area Determination Procedure
ter Functions
                                                         for Woven Geotextiles.
The primary geotextile characteristics influencing            (1) Installation of geotextile. A small section
filter functions are opening size (as related to soil    of the geotextile to be tested should be installed in

                                                                                                                         3-1
TM 5-818-8/AFJMAN 32-1030

a standard 2 by 2 inch slide cover, so that it can      one of the following conditions exists:
be put into a slide projector and projected onto a           (1) The soil is very widely graded, having a
screen. Any method to hold the geotextile section       coefficient of uniformity C greater than 20.
                                                                                   U
and maintain it perpendicular to the projected               (2) The soil is gap graded. (Soils lacking a
light can be used.                                      range of grain sizes within their maximum and
     (2) Slide projector. The slide projector should    minimum grain sizes are called “gap graded” or
be placed level to eliminate any distortion of the      “skip graded” soils.) Should these conditions exist
geotextile openings. After placing the slide in the     in combination with risk of extremely high repair
projector and focusing on a sheet of paper approxi-     costs if failure of the filtration system occurs the
mately 8 to 10 feet away, the opening outlines can      gradient ratio test may be required.
be traced.                                                 e. Clogging Resistance. Clogging is the reduc-
     (3) Representative area. Draw a rectangle of       tion in permeability or permittivity of a geotextile
about 0.5 to 1 square foot area on the “projection      due to blocking of the pores by either soil particles
screen” sheet of paper to obtain a representative       or biological or chemical deposits. Some clogging
area to test; then trace the outline of all openings    takes place with all geotextiles in contact with
inside the designated rectangle.                        soil. Therefore, permeability test results can only
     (4) Finding the area. After removing the           be used as a guide for geotextile suitability. For
sheet, find the area of the rectangle, using a          woven geotextiles, if the POA is sufficiently large,
planimeter. If necessary, the given area may be         the geotextiles will be resistant to clogging. The
divided to accommodate the planimeter.                  POA has proved to be a useful measure of clogging
     (5) Total area of openings. Find the total area    resistance for woven textiles but is limited to
of openings inside rectangle, measuring the area of     woven geotextiles having distinct, easily measured
each with a planimeter.                                 openings. For geotextiles which cannot be evalu-
     (6) Compute percent. Compute POA by the            ated by POA, soil- geotextile permeameters have
equation:                                               been developed for measuring soil-geotextile per-
                                                        meability and clogging. As a measure of the
             Total Area Occupied by Openings            degree to which the presence of geotextile affects
      POA=                                     x 100
               Total Area of Test Rectangle             the permeability of the soil- geotextile system, the
                                                        gradient ratio test can be used (ASTM D 5101).
   c. Flow Reversals. Piping criteria are based on      The gradient ratio is defined as the ratio of the
granular drain criteria for preventing drain mate-      hydraulic gradient across the geotextile and the 1
rial from entering openings in drain pipes. If flow     inch of soil immediately above the geotextile to
through the geotextile drain installation will be       the hydraulic gradient between 1 and 3 inches
reversing and/or under high gradients (especially       above the geotextile.
if reversals are very quick and involve large
changes in head), tests, modeling prototype condi-      3-5.   Permeability
tions, should be performed to determine geotextile        a. Transverse Permeability. After installation,
requirements.                                           geotextiles used in filtration and drainage applica-
   d. Clogging. There is limited evidence (Giroud       tions must have a flow capacity adequate to
1982) that degree of uniformity and density of          prevent significant hydrostatic pressure buildup in
granular soils (in addition to the D8 5 size) influ-    the soil being drained. This flow capacity must be
ence the ability of geotextiles to retain the drained   maintained for the range of flow conditions for
soil. For very uniform soils (uniformity coefficient    that particular installation. For soils, the indicator
2 to 4), the maximum AOS may not be as critical         of flow capacity is the coefficient of permeability
as for more well graded soils (uniformity coeffi-       as expressed in Darcy's Law (TM 5-820-2/
cient greater than 5). A gradient ratio test with       AFSMAN 32-1016 ). The proper application of
observation of material passing the geotextile may      Darcy’s Law requires that geotextile thickness be
be necessary to determine the adequacy of the           considered. Since the ease of flow through a
material. In normal soil- geotextile filter systems,    geotextile regardless of its thickness is. the prop-
detrimental clogging only occurs when there is          erty of primary interest, Darcy’s Law can be
migration of fine soil particles through the soil       modified to define the term permittivity, Ψ, with
                                                                    -1
matrix to the geotextile surface or into the geotex-    units of sec. , as follows:
tile. For most natural soils, minimal internal
migration will take place. However, internal mi-                                                     (eq 3-1)
gration may take place under sufficient gradient if


3-2
                                                                             TM 5-818-8/AFJMAN 32-1030

where                                                   gradients used, the normal pressure applied to the
                                                        product being tested, the potential for reduction of
                                                        transmissivity over time due to creep of the drain-
                                                        age material, and the possibility that intermittent
                                                        flow will result in only partial saturation of the
                                                        drainage material and reduced flow capacity.
                                                        ASTM D 4716 may be used for evaluating the
                                                        transmissivity of drainage materials.
The limitation of directly measuring the perme-            c. Limiting Criteria. Permeability criteria for
ability and permittivity of geotextiles is that         nonwoven geotextiles require that the permeabil-
Darcy’s Law applies only as long as laminar flow        ity of the geotextile be at least five times the
exists. This is very difficult to achieve for geotex-   permeability of the surrounding soil. Permeability
tiles since the hydraulic heads required to assure      criteria for woven geotextiles are in terms of the
laminar flow are so small that they are difficult to    POA. When the protected soil has less than 0.5
accurately measure. Despite the fact that Darcy’s       percent passing the No. 200 sieve, the POA should
equation does not apply for most measurements of        be equal to or greater than 10 percent. When the
permeability, the values obtained are considered        protected soil has more than 5 percent but less
useful as a relative measure of the permeabilities      than 85 percent passing the No. 200 sieve, the
and permittivities of various geotextiles. Values of    POA should be equal to or greater than 4 percent.
permeability reported in the literature, or obtained
from testing laboratories, should not be used with-     3-6. Other Filter Considerations
out first establishing the actual test conditions          a. To prevent clogging or blinding of the geotex-
used to determine the permeability value. ASTM          tile, intimate contact between the soil and geotex-
Method D 4491 should be used for establishing the       tile should be assured during construction. Voids
permeability and permittivity of geotextiles. The       between the soil and geotextile can expose the
permeability of some geotextiles decreases signifi-     geotextile to a slurry or muddy water mixture
cantly when compressed by surrounding soil or           during seepage. This condition promotes erosion of
rock. ASTM D 5493 can be used for measuring the         soil behind the geotextile and clogging of the
permeabilities of geotextiles under load.               geotextile.
   b. In-plane Permeability. Thick nonwoven geo-           b. Very fine-grained noncohesive soils, such as
textiles and special products as prefabricated          rock flour, present a special problem, and design of
drainage panels and strip drains have substantial       drain installations in this type of soil should be
fluid flow capacity in their plane. Flow capacity in    based on tests with expected hydraulic conditions
a plane of a geotextile is best expressed indepen-      using the soil and candidate geotextiles.
dently of the material’s thickness since the thick-        c. As a general rule slit-film geotextiles are
ness of various materials may differ considerably,      unacceptable for drainage applications. They may
while the ability to transmit fluid under a given       meet AOS criteria but generally have a very low
head and confining pressure is the property of          POA or permeability. The wide filament in many
interest. The property of in-plane flow capacity of     slit films is prone to move relative to the cross
a geotextile is termed “transmissivity,” θ , and is     filaments during handling and thus change AOS
expressed as:                                           and POA.
                                                           d. The designer must consider that in certain
                                             (eq 3-2)
                                                        areas an ochre formation may occur on the geotex-
                                                        tile. Ochre is an iron deposit usually a red or tan
where                                                   gelatinous mass associated with bacterial slimes.
                                                        It can, under certain conditions, form on and in
                                                        subsurface drains. The designer may be able to
                                                        determine the potential for ochre formation by
                                                        reviewing local experience with highway, agricul-
                                                        tural, embankment, or other drains with local or
                                                        state agencies. If there is reasonable expectation
                                                        for ochre formation, use of geotextiles is discour-
                                                        aged since geotextiles may be more prone to clog.
Certain testing conditions must be considered if        Once ochre clogging occurs, removal from geotex-
meaningful values of transmissivity are to be           tiles is generally very difficult to impossible, since
acquired. These conditions include the hydraulic        chemicals or acids used for ochre removal can-
                                                                                                         3-3
TM 5-818-8/AFJMAN 32-1030

damage geotextiles, and high pressure jetting                       placement. This can be done by using small
through the perforated pipe is relatively ineffec-                  amounts of aggregate to hold the geotextile in
tive on clogged geotextiles.                                        place or using loose pinning and repinning as
                                                                    necessary to keep the geotextile loose. This method
3-7. Strength Requirements                                          of placement will typically require 10 to 15 per-
Unless geotextiles used in drainage applications                    cent more geotextile than predicted by measure-
have secondary functions (separation, reinforce-                    ment of the drain’s planer surfaces.
ment, etc.) requiring high strength, the require-                      c. Joints.
ments shown in table 3-2 will provide adequate                            (1) Secure lapping or joining of consecutive
strength.                                                           pieces of geotextile prevents movement of soil into
                                                                    the drain. A variety of methods such as sewing,
    Table 3-2. Geotextile Strength Requirements for Drains.         heat bonding, and overlapping are acceptable
Strength Type    Test Method        Class A 1       Class B 2
                                                                    joints. Normally, where the geotextile joint will
                                                                    not be stressed after installation, a minimum
Grab Tensile     ASTM   D   4632      180               80          12-inch overlap is required with the overlapping
Seam             ASTM   D   4632      160               70
Puncture         ASTM   D   4833       80               25
                                                                    inspected to ensure complete geotextile-to-geo-
Burst            ASTM   D   3786      290              130          textile contact. When movement of the geotextile
Trapezoid Tear   ASTM   D   4533       50               25          sections is possible after placement, appropriate
1
                                                                    overlap distances or more secure joining methods
   Class A Drainage applications are for geotextile installation
where applied stresses are more severe than Class B applica-        should be specified. Field joints are much more
tions; i.e., very coarse shape angular aggregate is used, compac-   difficult to control than those made at the factory
tion is greater than 95 percent of ASTM D 1557 of maximum           or fabrication site and every effort should be made
density or depth of trench is greater than 10 feet.                 to minimize field joining.
2 Class B Drainage applications are for geotextile installations
                                                                          (2) Seams are described in chapter 1. Strength
where applied stresses are less severe than Class A applica-
tions; i.e., smooth graded surfaces having no sharp angular
                                                                    requirements for seams may vary from just
projections, and no sharp angular aggregate, compaction is less     enough to hold the geotextile sections together for
than or equal to 95 percent of ASTM D 1557 maximum density.         installation to that required for the geotextile.
                                                                    Additional guidance for seams is contained in
3-8. Design and Construction Considerations                         AASHTO M 288. Seam strength is determined
   a. Installation Factors. In addition to the re-                  using ASTM 4632.
quirement for continuous, intimate geotextile con-                     d. Trench Drains.
tact with the soil, several other installation factors                    (1) Variations of the basic trench drain are
strongly influence geotextile drain performance.                    the most common geotextile drain application.
These include:                                                      Typically, the geotextile lines the trench allowing
     (1) How the geotextile is held in place during                 use of a very permeable backfill which quickly
construction.                                                        removes water entering the drain. Trench drains
     (2) Method of joining consecutive geotextile                    intercept surface infiltration in pavements and
elements.                                                            seepage in slopes and embankments as well as
     (3) Preventing geotextile contamination.                        lowering ground-water levels beneath pavements
     (4) Preventing geotextile deterioration from                    and other structures. The normal construction
exposure to sunlight. Geotextile should retain 70                    sequence is shown in figure 3-l. In addition to
percent of its strength after 150 hours of exposure                 techniques shown in figure 3-1, if high compactive
to ultraviolet sunlight (ASTM D 4355).                               efforts are required (e.g., 95 percent of ASTM D
   b. Placement. Pinning the geotextile with long                    1557 maximum density), the puncture strength
nail-like pins placed through the geotextile into                    requirements should be doubled. Granular backfill
the soil has been a common method of securing the                    does not have to meet piping criteria but should be
geotextile until the other components of the drain                   highly permeable, large enough to prevent move-
have been placed; however, in some applications,                     ment into the pipe, and meet durability and
this method has created problems. Placement of                       structural requirements of the project. This allows
aggregate on the pinned geotextile normally puts                     the designer to be much less stringent on backfill
the geotextile into tension which increases poten-                   requirements than would be necessary for a totally
tial for puncture and reduces contact of the geotex-                 granular trench drain. Some compaction of the
tile with soil, particularly when placing the geo-                   backfill should always be applied.
textile against vertical and/or irregular soil                            (2) Wrapping of the perforated drain pipe with
surfaces. It is much better to keep the geotextile                   a geotextile when finer grained filter backfill is
loose but relatively unwrinkled during aggregate                     used is a less common practice. Normally not used

3-4
                                                                              TM 5-818-8/AFJMAN 32-1030




                    TRENCH EXCAVATED AND                       BEDDING (USUALLY 6-INCH
                    GEOTEXTILE PLACED TO                       MINIMUM) AND COLLECTOR
                    INSURE INTIMATE CONTACT                    PIPE PLACED (IF PIPE IS
                    WITH SOIL SURFACES AND                     REQUIRED)
                    THAT PROPER OVERLAP WILL
                    BE AVAILABLE AFTER BACK-
                    FILLING




                   REMAINDER OF BACKFILL                          GEOTEXTILE SECURELY OVER-
                   PLACED AND COMPACTED AS                        LAPPED (USUALLY 12-INCH
                   REQUIRED TO PRODUCE COM-                       MINIMUM) ABOVE BACKFILL
                   PATIBLE STRENGTH AND                           SO SOIL INFILTRATION IS
                   CONSOLIDATION WITH SUR-                        PREVENTED. COVER MATE-
                   ROUNDING SOIL AND STRUCTURES                   RIAL PLACED AND COMPACTED

                                    Figure 3-1. Trench Drain Construction.
in engineered applications, this method is less           as a cover for the pipe perforations preventing
efficient than lining the trench with a geotextile        backfill infiltration. If the geotextile can be sepa-
because the reduced area of high permeability             rated a small distance from the pipe surface, the
material concentrates flow and lowers drain eff-          flow through the geotextile into the pipe openings
ciency. Wrapping of the pipe may be useful when           will be much more efficient. Use of plastic corru-
finer grained filter materials are best suited be-        gated, perforated pipe with openings in the de-
cause of availability and/or filter grain size re-        pressed portion of the corrugation is an easy way
quirements. In this case, the geotextile functions        of doing this.




                                                                                                             3-5
                                                                              TM 5-818-8/AFJMAN 32-1030

                                               CHAPTER 4

          GEOTEXTILE REINFORCED EMBANKMENT ON SOFT FOUNDATION

4-1. General                                            and the dike, the geotextile will increase the
Quite often, conventional construction techniques       resisting forces of the foundation. Geotextile-
will not allow dikes or levees to be constructed on     reinforced dikes may fail by fill material sliding
very soft foundations because it may not be cost        off the geotextile surface, geotextile tensile failure,
effective, operationally practical, or technically      or excessive geotextile elongation. These failures
feasible. Nevertheless, geotextile-reinforced dikes     can be prevented by specifying the geotextiles that
have been designed and constructed by being made        meet the required tensile strength, tensile modu-
to float on very soft foundations. Geotextiles used     lus, and soil-geotextile friction properties.
in those dikes alleviated many soft-ground founda-         e. Rotational Slope and/or Foundation Failure.
tion dike construction problems because they per-       Geotextile-reinforced dikes constructed to a given
mit better equipment mobility, allow expedient          height and side slope will resist classic rotational
construction, and allow construction to design ele-     failure if the foundation and dike shear strengths
vation without failure. This chapter will address       plus the geotextile tensile strength are adequate
the potential failure modes and requirements for        (fig 4-l b). The rotational failure mode of the dike
design and selection of geotextiles for reinforced      can only occur through the foundation layer and
embankments.                                            geotextile. For cohesionless fill materials, the dike
                                                        side slopes are less than the internal angle of
4-2. Potential Embankment Failure Modes                 friction. Since the geotextile does not have flexural
The design and construction of geotextile-rein-         strength, it must be placed such that the critical
forced dikes on soft foundations are technically        arc determined from a conventional slope stability
feasible, operationally practical, and cost effective   analysis intercepts the horizontal layer. Dikes
when compared with conventional soft foundation         constructed on very soft foundations will require a
construction methods and techniques. To success-        high tensile strength geotextile to control the
fully design a dike on a very soft foundation, three    large unbalanced rotational moments.
potential failure modes must be investigated (fig          f. Excessive Vertical Foundation Displacements.
4-1).                                                   Consolidation settlements of dike foundations,
   a. Horizontal sliding, and spreading of the em-      whether geotextile-reinforced or not, will be simi-
bankment and foundation.                                lar. Consolidation of geotextile-reinforced dikes
   b. Rotational slope and/or foundation failure.       usually results in more uniform settlements than
   c. Excessive vertical foundation displacement.       for non-reinforced dikes. Classic consolidation
The geotextile must resist the unbalanced forces        analysis is a well-known theory, and foundation
necessary for dike stability and must develop           consolidation analysis for geotextile-reinforced
moderate-to-high tensile forces at relatively low-to-   dikes seems to agree with predicted classical con-
moderate strains. It must exhibit enough soil-          solidation values. Soft foundations may fail par-
fabric resistance to prevent pullout. The geotextile    tially or totally in bearing capacity before classic
tensile forces resist the unbalanced forces, and its    foundation consolidation can occur. One purpose of
tensile modulus controls the vertical and horizon-      geotextile reinforcement is to hold the dike to-
tal displacement of dike and foundation. Adequate       gether until foundation consolidation and strength
development of soil-geotextile friction allows the      increase can occur. Generally, only two types of
transfer of dike load to the geotextile. Developing     foundation bearing capacity failures may occur-
geotextile tensile stresses during construction at      partial or center-section foundation failure and
small material elongations or strains is essential.     rotational slope stability/foundation stability. Par-
   d. Horizontal Sliding and Spreading. These           tial bearing failure, or “center sag” along the dike
types of failure of the dike and/or foundation may      alignment (fig 4-1 c), may be caused by improper
result from excessive lateral earth pressure (fig       construction procedure, like working in the center
4-1a). These forces are determined from the dike        of the dike before the geotextile edges are covered
height, slopes, and fill material properties. During    with fill materials to provide anchorage. If this
conventional construction the dikes would resist        procedure is used, geotextile tensile forces are not
these modes of failure through shear forces devel-      developed and no benefit is gained from the geo-
oped along the dike-foundation interface. Where         textile used. A foundation bearing capacity failure
geotextiles are used between the soft foundation        may occur as in conventional dike construction.

                                                                                                         4-1
TM 5-818-8/AFJMAN 32-1030




                             a POTENTIAL EMBANKMENT FAILURE FROM
                                    LATERAL EARTH PRESSURE




                              b. POTENTIAL EMBANKMENT ROTATIONAL
                                    SLOPE/FOUNDATION FAILURE




                          c. POTENTIAL EMBANKMENT FAILURE FROM
                                  EXCESSIVE DISPLACEMENT
                     Figure 4-1. Potential Geotextile-Reinforced Embankment Failure Modes.




4-2
                                                                            TM 5-818-8/AFJMAN 32-1030

Center sag failure may also occur when low-tensile     using assumed logarithmic spiral or circular fail-
strength or low-modulus geotextiles are used, and      ure surfaces. Another bearing capacity failure is
embankment spreading occurs before adequate            the possibility of lateral squeeze (plastic flow) of
geotextile stresses can be developed to carry the      the underlying soils. Therefore, the lateral stress
dike weight and reduce the stresses on the founda-     and corresponding shear forces developed under
tion. If the foundation capacity is exceeded, then     the embankment should be compared with the
the geotextile must elongate to develop the re-        sum of the resisting passive forces and the product
quired geotextile stress to support the dike weight.   of the shear strength of the soil failure plane area.
Foundation bearing-capacity deformation will oc-       If the overall bearing capacity analysis indicates
cur until either the geotextile fails in tension or    an unsafe condition, stability can be improved by
carries the excess load. Low modulus geotextiles       adding berms or by extending the base of the
generally fail because of excessive foundation dis-    embankment to provide a wide mat, thus spread-
placement that causes these low tensile strength       ing the load to a greater area. These berms or
geotextiles to elongate beyond their ultimate          mats may be reinforced by properly designing
strength. High modulus geotextiles may also fail if    geotextiles to maintain continuity within the em-
their strength is insufficient. This type of failure   bankment to reduce the risk of lateral spreading.
may occur where very steep dikes are constructed,      Wick drains may be used in case of low bearing
and where outside edge anchorage is insufficient.      capacity to consolidate the soil rapidly and achieve
4-3. Recommended Criteria                              the desired strength. The construction time may
                                                       be expedited by using geotextile reinforcement.
The limit equilibrium analysis is recommended for
                                                          b. Slope Stability Analysis. If the overall bear-
design of geotextile-reinforced embankments.
                                                       ing capacity of the embankment is determined to
These design procedures are quite similar to con-
                                                       be satisfactory, then the rotational failure poten-
ventional bearing capacity or slope stability analy-
                                                       tial should be evaluated with conventional limit
sis. Even though the rotational stability analysis
assumes that ultimate tensile strength will occur      equilibrium slope stability analysis or wedge anal-
instantly to resist the active moment, some geotex-    ysis. The potential failure mode for a circular arc
tile strain, and consequently embankment dis-          analysis is shown in figure 4-2. The circular arc
placement, will be necessary to develop tensile        method simply adds the strength of the geotextile
stress in the geotextile. The amount of movement       layers to the resistance forces opposing rotational
within the embankment may be limited by the use        sliding because the geotextile must be physically
of high tensile modulus geotextiles that exhibit       torn for the embankment to slide. This analysis
good soil-geotextile frictional properties. Conven-    consists of determining the most critical failure
tional slope stability analysis assumes that the       surfaces, then adding one or more layers of geotex-
geotextile reinforcement acts as a horizontal force    tile at the base of the embankment with sufficient
to increase the resisting moment. The following        strength at acceptable strain levels to provide the
analytical procedures should be conducted for the      necessary resistance to prevent failure at an ac-
design of a geotextile-reinforced embankment: (1)      ceptable factor of safety. Depending on the nature
overall bearing capacity, (2) edge bearing capacity    of the problem, a wedge-type slope stability analy-
or slope stability, (3) sliding wedge analysis for     sis may be more appropriate. The analysis may be
embankment spreading/splitting, (4) analysis to        conducted by accepted wedge stability methods,
limit geotextile deformation, and (5) determine        where the geotextile is assumed to provide hori-
geotextile strength in a direction transverse to the   zontal resistance to outward wedge sliding and
longitudinal axis of the embankment or the longi-      solving for the tensile strength necessary to give
tudinal direction of the geotextile. In addition,      the desired factor of safety. The critical slip circle
embankment settlements and creep must also be          or potential failure surfaces can be determined by
considered in the overall analysis.                    conventional geotechnical limited equilibrium
   a. Overall Bearing Capacity. The overall bearing    analysis methods. These methods may be simpli-
capacity of an embankment must be determined           fied by the following assumptions:
whether or not geotextile reinforcement is used. If         (1) Soil shear strength and geotextile tensile
the overall stability of the embankment is not         strength are mobilized simultaneously.
satisfied, then there is no point in reinforcing the        (2) Because of possible tensile crack forma-
embankment. Several bearing capacity procedures        tions in a cohesionless embankment along the
are given in standard foundation engineering text-     critical slip surface, any shear strength developed
books. Bearing capacity analyses follow classical      by the embankment (above the geotextile) should
limiting equilibrium analysis for strip footings,      be neglected.

                                                                                                         4-3
TM 5-818-8/AFJMAN 32-1030




             Figure 4-2. Concept Used for Determining Geotextile Tensile Strength Necessary to Prevent Slope Failure.

     (3) The conventional assumption is that criti-                 in figure 4-3. These forces consist of an actuating
cal slip circles will be the same for both the                      force composed of lateral earth pressure and a
geotextile-reinforced and nonreinforced embank-                     resisting force created by frictional resistance be-
ments although theoretically they may be differ-                    tween the embankment fill and geotextile. To
ent. Under these conditions, a stability analysis is                provide the adequate resistance to sliding failure,
performed for the no-geotextile condition, and a                    the embankment side slopes may have to be
critical slip circle and minimum factor of safety is                adjusted, and a proper value of soil-geotextile
obtained. A driving moment or active moment                         friction needs to be selected. Lateral earth pres-
(AM) and soil resistance moment (RM) are deter-                     sures are maximum beneath the embankment
mined for each of the critical circles. If the factor               crest. The resultant of the active earth pressure
of safety (FS) without geotextile is inadequate,                    per unit length       for the given cross section
then an additional reinforcement resistance mo-                     may be calculated as follows:
ment can be computed from the following equa-                                                                           (eq 4-2)
tion:
                                                                    where
TR + RM/FS = AM                                     (eq 4-1)                 = embankment fill compacted density-force
where                                                                          per length cubed
       T = geotextile tensile strength                                     H = maximum embankment height
       R = radius of critical slip circle                                    = coefficient of active earth pressure (di-
      RM = soil resistance moment                                              mensionless)
      FS = factor of safety                                         For a cohesionless embankment fill, the equation
      AM = driving or active moment                                 becomes:
This equation can be solved for T so that the
                                                                                                                        (eq 4-3)
geotextile reinforcement can also be determined to
provide the necessary resisting moment and re-
                                                                    Resistance to sliding may be calculated per unit
quired FS.
                                                                    length of embankment as follows:
  c. Sliding Wedge Analysis. The forces involved
in an analysis for embankment sliding are shown                                                                         (eq 4-4)



4-4
                                                                                       TM 5-818-8/AFJMAN 32-1030




                     a. FORCES INVOLVED IN SPLITTING AND SLIDING ANALYSES




          NOTE:   FABRIC MODULES CONTROLS
                  LATERAL SPREADING


                     b. GEOTEXTILE STRAIN CHARACTERISTICS RELATING TO
                              EMBANKMENT SPREADING ANALYSIS
                      Figure 4-3. Assumed Stresses and Strains Related to Lateral Earth Pressures.

where                                                          given embankment geometry the FS is controlled
    PR = resultant of resisting forces                         by the soil-geotextile friction. A minimum FS of
      X = dimensionless slope parameter (i.e., for             1.5 is recommended against sliding failure. By
          3H on 1V slope, X = 3 or an average                  combining the previous equations with a factor of
          slope may be used for different embank-              2, and solving for     , the soil geotextile friction
          ment configurations)                                 angle gives the following equation:
        = soil-geotextile friction angle (degrees)
                                                                                                          (eq 4-5)
A factor of safety against embankment sliding
failure may be determined by taking the ratio of               If it is determined that the required soil-geotextile
the resisting forces to the actuating forces. For a            friction angle exceeds what might be achieved
                                                                                                                4-5
TM 5-818-8/AFJMAN 32-1030

with the soil and geotextile chosen, then the           as the average strain, then the maximum strain
embankment side slopes must be flattened, or            which would occur is 5 percent.
additional berms may be considered. Most high-             e. Potential Embankment Rotational Displace-
strength geotextiles exhibit a fairly high soil-        ment. It is assumed that the geotextile ultimate
geotextile friction angle that is equal to or greater   tensile resistance is instantaneously developed to
than 30 degrees, where loose sand-size fill material    prevent rotational slope/foundation failure and is
is utilized. Assuming that the embankment sliding       inherently included in the slope stability limit
analysis results in the selection of a geotextile       equilibrium analysis. But for the geotextile to
that prevents embankment fill material from slid-       develop tensile resistance, the geotextile must
ing along the geotextile interface, then the result-    strain in the vicinity of the potential failure plane.
ant force because of lateral earth pressure must be     To prevent excessive rotational displacement, a
less than the tensile strength at the working load      high-tensile-modulus geotextile should be used.
of the geotextile reinforcement to prevent spread-      The minimum required geotextile tensile modulus
ing or tearing. For an FS of 1, the tensile strength    to limit or control incipient rotational displace-
would be equal to the resultant of the active earth     ment is the same as for preventing spreading
pressure per unit length of embankment. A mini-         failure.
mum FS of 1.5 should be used for the geotextile to         f. Longitudinal Geotextile Strength Require-
prevent embankment sliding. Therefore, the mini-        ments. Geotextile strength requirements must be
mum required tensile strength to prevent sliding        evaluated and specified for both the transverse
is:                                                     and longitudinal direction of the embankment.
                                                        Stresses in the warp direction of the geotextile or
   = 1.5 P A                                 (eq 4-6)   longitudinal direction of the embankment result
where      = minimum geotextile tensile strength.       from foundation movement where soils are very
                                                        soft and create wave or a mud flow that drags on
  d. Embankment Spreading Failure Analysis.             the underside of the geotextile. The mud wave not
Geotextile tensile forces necessary to prevent lat-     only drags the geotextile in a longitudinal direc-
eral spreading failure are not developed without        tion but also in a lateral direction toward the
some geotextile strain in the lateral direction of      embankment toes. By knowing the shear strength
the embankment. Consequently, some lateral              of the mud wave and the length along which it
movement of the embankment must be expected.            drags against the underneath portion of the geo-
Figure 4-3 shows the geotextile strain distribution     textile, then the spreading force induced can be
that will occur from incipient embankment spread-       calculated. Forces induced during construction in
ing if it is assumed that strain in the embankment      the longitudinal direction of the embankment may
varies linearly from zero at the embankment toe         result from the lateral earth pressure of the fill
to a maximum value beneath embankment crest.            being placed. These loads can be determined by
Therefore, an FS of 1.5 is recommended in deter-        the methods described earlier where
mining the minimum required geotextile tensile           and     = 20     at 5 percent strain. The geotextile
modulus. If the geotextile tensile strength              strength required to support the height of the
determined by equation 4-6 is used to determine          embankment in the direction of construction must
the required tensile modulus           an FS of 1.5      also be evaluated. The maximum load during
will be automatically taken into account, and the        construction includes the height or thickness of
minimum required geotextile tensile modulus may         the working table, the maximum height of soil and
be calculated as follows:                               the equipment live and dead loads. The geotextile
                                                         strength requirements for these construction loads
                                             (eq 4-7)    must be evaluated using the survivability criteria
                                                         discussed previously.
where        = maximum strain which the geotex-            g. Embankment Deformation. One of the pri-
tile is permitted to undergo at the embankment           mary purposes of geotextile reinforcement in an
center line. The maximum geotextile strain is            embankment is to reduce the vertical and horizon-
equal to twice the average strain over the embank-       tal deformations. The effect of this reinforcement
ment width. A reasonable average strain value of         on horizontal movement in the embankment
2.5 percent for lateral spreading is satisfactory        spreading modes has been addressed previously.
from a construction and geotextile property stand-       One of the more difficult tasks is to estimate the
point. This value should be used in design but           deformation or subsidence caused by consolidation
depending on the specific project requirements           and by plastic flow or creep of very soft foundation
larger strains may be specified. Using 2.5 percent       materials. Elastic deformations are a function of
4-6
                                                                                     TM 5-818-8/AFJMAN 32-1030

 the subgrade modulus. The presence of a geotextile           from consolidation settlements, plastic creep, and
 increases the overall modulus of the reinforced              flow of the soft foundation materials will be
 embankment. Since the lateral movement is mini-              minimized. It is recommended that a conventional
 mized by the geotextile, the applied loads to the            consolidation analysis be performed to determine
 soft foundation materials are similar to the ap-             foundation settlements.
 plied loads in a laboratory consolidation test.              4-4. Example Geotextile-Reinforced Embank-
 Therefore, for long-term consolidation settlements           ment Design
 beneath geotextile-reinforced embankments, the
                                                                 a. The Assumption.
 compressibility characteristics of the foundation
                                                                   (1) An embankment, fill material consisting of
 soils should not be altered by the presence of the           clean sand with        = 100 pounds per cubic foot,
 reinforcement. A slight reduction in total settle-           and φ = 30 degrees (where φ is the angle of
 ment may occur for a reinforced embankment but               internal friction).
 no significant improvement. Other studies indicate                (2) Foundation properties (unconsolidated, un-
 that very high-strength, high-tensile modulus geo-           determined shear strength) as shown in figure 4-4
 textiles can control foundation displacement dur-            (water table at surface).
 ing construction, but the methods of analysis are                 (3) Embankment dimensions (fig 4-4).
 not as well established as those for stability                       (a) Crest width of 12 feet.
 analysis. Therefore, if the embankment is designed                   (b) Embankment height (H) of 7 feet.
 for stability as outlined previously, then the lat-                  (c) Embankment slope, 10 Horizontal on 1
 eral and vertical movements caused by subsidence             Vertical (i.e., x = 10).


NOTE:   NATURAL GROUND SURFACE COVERED
        WITH GRASS AND VOID OF OTHER THAN
        SMALL DEBRIS, HUMPS, DEPRESSIONS,
        ETC. MAY OR MAY NOT HAVE A CRUST




             Figure 4-4. Embankment Section and Foundation Conditions of Embankment Design Example Problem.


                                                                                                              4-7
TM 5-818-8/AFJMAN 32-1030

  b. Factor of Safety. This design example will        4-5.   Bearing-Capacity       Consideration
consider an FS of 1.3 against rotational slope         A second bearing-capacity consideration is the
failure, 1.5 against spreading, 2.0 against sliding    chance of soft foundation material squeezing out.
failure, and 1.3 against excessive rotational dis-     Therefore, the lateral stress and corresponding
placement for the geotextile fabric requirements.      shear forces below the embankment, with respect
Determine minimum geotextile requirements.             to resisting passive forces and shear strength of
  c. Calculate Overall Bearing Capacity.               soil, are determined.
     (1) Ultimate bearing capacity qult for strip        a. Plastic flow method for overall squeeze-
footing on clay.                                       squeeze between two plates.
                 = (75)(5.14) = 385 pounds per
                                square foot (with             =                                       (eq 4-8)
                                surface crust)                    2L + crest width
                 = (75)(3.5) = 263 pounds per          where
                               square foot (without        c = cohesion (shear strength) of soil
                               surface crust)              a = ½ distance between embankment and
Values shown for         are standard values for φ =           next higher strength foundation soil layer
0. It has been found from experience that excessive       L = width of embankment slope
mud wave formation is minimized when a dried           For the conditions in previous example:
crust has formed on the ground surface.
     (2) Applied stress.                                                       (700) 14
                                                                                      2
                                                                                        ( )
                = lOO(7) = 700 pounds per square
                           foot                                                   140 + 12

    (3) Determine FS. The bearing capacity was                               = 32.2
not sufficient for an unreinforced embankment,
but for a geotextile-reinforced embankment, the        Cohesion available is 75 pounds per square foot,
lower portion of its base will act like a mat          which is greater than 32.2 pounds per square foot
foundation, thus distributing the load uniformly       required and is therefore satisfactory.
over the entire embankment width. Then, the              b. Toe squeeze of soft foundation materials is a
average vertical applied stress is:                    common problem that requires investigating.
                                                       Therefore, the passive resistance for toe squeeze is
                                                       as follows:
                                                          (just below embankment) =
                                                                                                      (eq 4-9)

                                                                                                     (eq 4-10)
                                                       Then, the difference:
                    2 x 70 + 12
                                                                                                     (eq 4-11)
                        = 378                                                                        (eq 4-12)
                                                       For the example:
             FS =          378 < 1 . 0
                           385                                    = 4(75) - 378
                                                                  = 300 - 378
where L = width of embankment slope. If a dried
                                                                  = 78
crust is available on the soft foundation surface,
then the FS is about 1. If no surface crust is             is greater than ; therefore, foundation
available, the FS is less than 1.0, and the embank-    squeeze may occur. Solutions would be to either
ment slopes or crest height would have to be           allow squeezing to occur or construct shallow
modified. Since the embankment is very wide and        berms to stabilize the embankment toe or use
the soft clay layer is located at a shallow depth,     plastic strip drains.
failure is not likely because the bearing-capacity        c. Slope Stability Analysis. Perform a slope sta-
analysis assumes a uniform soil twice the depth of     bility analysis to determine the required geotextile
the embankment width.                                  tensile strength and modulus to provide an FS of
4-8
                                                                            TM 5-818-8/AFJMAN 32-1030

1.3 against rotational slope failure. There are                L =      = 2,800
many slope stability procedures available in the                             287
literature for determining the required tensile
                                                               L = 9.8 ft; approximately 10 ft
strength T . Computer programs are also available
that will determine the critical slip surface with a      e. Prevention of Sliding. Calculate       to pro-
search routine. Assume that an analysis was             vide an FS of 2 against sliding failure across the
conducted on the example embankment and an              geotextile.
active moment of 840,000 foot-pounds per foot of            (1) Calculate lateral earth pressure,
width was calculated and a resisting moment of
820,000 foot-pounds per foot of width calculated for
a slip circle having a radius of 75 feet. This would
result in a safety factor of 0.98 which is not
satisfactory. Using equation 4-1, the tensile
strength of a geotextile necessary to provide an FS
of 1.3 can be calculated as follows:                           PA = 8 1 7
                                                        (2) Calculate
       AM - RM
            FS                                                 FS = Resisting Force
   T =
         R                                                              Active Force


            840,000 -820,000
                                                               FS =
   T =                  1.3 = 2,800 pounds per
                  75             foot of width

   d. Pullout Resistance. Pullout resistance of the
geotextile from the intersection of the potential
failure plane surface is determined by calculating
the resistance and necessary geotextile embedment       where X = ratio of the vertical and horizonal slope
length. There are two components to geotextile          (i.e., 10 horizontal to 1 vertical).
pullout resistance-one below and one above the
geotextile. Resistance below the geotextile in this
example is 50 pounds per square foot, and resis-
tance above the geotextile is determined by the
average height of fill above the geotextile in the
affected areas. In this example, the resistance
above and below the geotextile is determined as
follows:

                                            (eq 4-13)     f. Prevention of Geotextile Splitting. Calculate
                                                        required geotextile tensile strength     to provide
where                                                   an FS of 1.5 against splitting.
          = moist weight of sand fill, 100 pounds per
            cubic foot                                         FS = 1.5 against splitting
        h = average height of sand fill above geotex-             = 817 pounds per foot width
            tile in the affected area, 6.5 feet         Calculate
         = sand-geotextile friction equal to
          = remolded strength of foundation clay
            soil beneath the geotextile, 50 pounds               = (1.5)(817)
            per square foot                                       = 1,226 pounds per foot width or
                                                                  = 102 pounds per inch width

             = 287 width                                  g. Limiting Spreading and Rotation. Calculate
                                                        the tensile modulus     required to limit embank-
The required pullout length is determined from          ment average spreading and rotation to 5 percent
the ultimate tensile strength requirement of 2,800      geotextile elongation.
pounds per foot width. Therefore,                           (1) Spreading analysis:
                                                                                                       4-9
TM 5-818-8/AFJMAN 32-1030

                                                               = remolded shear strength of foundation
            = (20)(102)                                materials
            = 2,040 pounds per inch width                  (2) Geotextile fill and seam tensile modulus of
       (2) Rotational slope stability analysis:        10 percent elongation:
              = 20 T
               = (20)(T = 233 pounds per inch width)
             = 4,670 pounds per inch width
                                                            i. Summary of Minimum Geotextile Require-
   h. Tensile Seam Strength and Fill Require-           ments. If the geotextile chosen is a woven polyes-
ments. Determine geotextile tensile strength re-       ter yarn and only 50 percent of the ultimate
quirements in geotextile till (cross machine direc-    geotextile load is used, then the minimum ulti-
tion) and across seams. Tensile strength               mate strength is 2 times the required working
requirement in this direction depends on the           tensile strength 233, or 466 ponds per inch width
amount of squeezing out and dragging loads on the      to compensate for possible creep.
underside of the geotextile and the amount of               (1) Soil-geotextile friction angle,      equals
shoving or sliding that the 2 to 3 feet of sand fill   3.9 degrees.
material causes during initial placement. If three          (2) Ultimate tensile strength        in the geo-
panels 16 feet wide are in place and the founda-       textile warp directions working tensile strength
tion material moves longitudinally along the em-       equals 466 pounds per inch width.
bankment alignment because of construction activ-            (3) Ultimate tensile strength       in the geo-
ities when establishing a working platform, then       textile fill and cross seams directions equals to 300
the loads in the geotextile fill direction can be      pounds per inch width.
calculated as follows:                                       (4) Tensile modulus (slope of line drawn
     (1) Geotextile fill and seam tensile strength     through zero load and strain and trough load at 5
requirement:                                           percent elongation) at 5 percent geotextile elonga-
                                                       tion in geotextile warp direction is 4,670 pounds
               = (3 panels) (l6 feet wide)
                                                       per inch width, (based on working tensile strength)
where                                                  and 10 percent geotextile elongation in the fill and
               = (3)(16 feet) (50 pounds per square    cross seam directions is 3,000 pounds per inch
                   foot.)                              width.
                = 2,400 pounds per foot width                (5) Contractor survivability and constructabi-
                = 200 pounds per inch width            lity requirements are included in tables 2-3, 2-4
                 at FS of 1.5 = 300 pounds per inch    and 2-5. Geotextile specifications must meet or
                   width                               exceed these requirements.




4-10
                                                                               TM 5-818-8/AFJMAN 32-1030

                                                CHAPTER 5

                RAILROAD TRACK CONSTRUCTION AND REHABILITATION


5-1. General                                              the track will be worsened. In any track construc-
                                                          tion or rehabilitation project, adequate drainage
The use of geotextiles in a railroad track structure
                                                          must be incorporated in the project design.
is dependent upon many factors including the
traffic, track structure, subgrade conditions, drain-     5-2. Material Selection
age conditions, and maintenance requirements. In             a. Based on current knowledge, woven geotex-
railroad applications, geotextiles are primarily          tiles are not recommended for use in railroad track
used to perform the functions of separation, filtra-      applications. Test installations have shown that
tion, and lateral drainage. Based on current              woven geotextiles tend to clog with time and act
knowledge, little is known of any reinforcement           almost as a plastic sheet preventing water from
effect geotextiles have on soft subgrades under           draining out of the subgrade.
railroad track. Therefore, geotextiles should not be         b. Geotextiles selected for use in the track struc-
used to reduce the ballast or subballast design           ture of military railroads should be nonwoven,
thickness. Geotextiles have found their greatest          needle-punched materials that meet the require-
railroad use in those areas where a large amount          ments listed in table 5-l.
of track maintenance has been required on an                 c. ASTM D 4886 is used to measure the abra-
existing right-of-way as a result of poor drainage        sion resistance of a geotextile for use in a railroad
conditions, soft conditions, and/or high-impact           application. Indications are that abrasion is
loadings. Geotextiles are normally placed between         greater for geotextiles placed during track rehabil-
the subgrade and ballast layer or between the             itations where the rail remains in-place than for
subgrade and subballast layers if one is present. A       geotextiles placed during new construction or reha-
common geotextile application is found in what is         bilitations where the existing rail, ties, and ballast
commonly known as “pumping track” and “ballast            are removed and the subgrade reworked. This may
pocket areas.” Both are associated with fine-             be due to the differences in the surface upon which
 grained subgrade soil and difficult drainage condi-      the geotextile is placed. In new construction the
 tions. Under traffic, transient vertical stresses are     subgrade surface is normally graded, compacted
 sufficient to cause the subgrade and ballast or           and free from large stone. During in-place rehabil-
 subballast materials to intermix if the subgrade is       itations the old ballast may be removed by under-
 weak (i.e. wet). As the intermixing continues, the        cutting or ploughing which leave ballast particles
 ballast becomes fouled by excessive fines contami-        loose on, or protruding from, the surface, creating
 nation, and a loss of free drainage through the           a rough surface for placement of the geotextile.
 ballast occurs as well as a loss of shear strength.
 The ballast is pulled down into the subgrade. As         5-3.   Application
 this process continues, ballast is forced deeper and     Geotextiles should be used to separate the ballast
 deeper into the subgrade, forming a pocket of            or subballast from the subgrade (or ballast from
 fouled and ineffective ballast and loss of track         subballast) in a railroad track in cut sections
 grade control. Ballast pockets tend to collect wa-       where the subgrade soil contains more than 25
 ter, further reducing the strength of the roadbed        percent by weight of particles passing the No. 200
 around them and result in continual track mainte-        sieve. Geotextiles are also used in embankment
 nance problems. Installation of geotextiles during       sections consisting of such material where there is
 rehabilitation of these areas provides separation,       less than 4 feet from the bottom of the tie to the
 filtration, and drainage functions and can prevent       ditch invert or original ground surface.
 the reoccurrence of pumping track. Common loca-
 tions for the installation of a geotextile in railroad   5-4. Depth of Placement
 track are locations of excessive track maintenance       Technical Manual TM 5-850-2/AFM 88-7, chap.
 resulting from poor subgrade/drainage conditions,        2 specifies a minimum ballast thickness of 12
 highway-railroad grade crossing, diamonds (rail-         inches. An additional minimum of 6 inches of
 road crossings), turnouts, and bridge approaches. If     subballast may be used in areas where drainage is
 a geotextile is installed in track without provisions    difficult. The actual total ballast/ subballast thick-
 made for adequate drainage, water will be re-            ness required is a function of the maximum wheel
 tained in the track structure and the instability of     load, rail weight, size, tie spacing, and allowable

                                                                                                            5-l
TM 5-818-8/AFJMAN 32-1030



                                                                                        1
                              Property                         Minimum Requirement                Test Method
                                (1)                                      (2)                          (3)

            Weight', ounce per square yard                                   15                 ASTM D 3776
                                                                                                 option B

            Structure                                         Needle-punched nonwoven                   --

            Grab tensile strength, pounds                                   350                  ASTM D 4632

            Elongation at failure, percent                                   20                  ASTM D 4632

            Burst strength, pounds per                                     620                   ASTM D 3786
             square inch

            Puncture strength, pounds                                      185                   ASTM D 4833

            Trapezoidal tear strength, pounds                              150                   ASTM D 4533

            Apparent opening size (AOS),                                 <0.22                   ASTM D 4751
             millimeter                                              (No. 70 sieve)

            Normal permeability,          , centimeters                     0.1                 ASTM D 4491
             per second

            Permittivity, seconds                                           0.2                 ASTM D 4491
                                                     4
            Planar water flow/transmissivity                                      6             ASTM D 4716
                                         -3
             square feet per minute X 10

            Ultraviolet degradation at 150 hours                              70                ASTM D 4355
             percent strength retained
                                      5
            Seam strength, pounds                                           350                 ASTM D 1683

             Value in weaker principal direction. All numerical values represent minimum
           average roll value.
            2
             The minimum weight listed herein is based on the experience that geotextiles
           with weights less than 15 oz/yd tend to show greater abrasion and wear than do
           heavier weight materials.   It is recommended that the selection of geotextile
           be based on the minimum physical property requirements of this table and not
           solely on weight.
           3
             The k of the geotextile should be at least five times greater than the k
           value of the soil.
           4
             Planar water flow/transmissivity determined at normal stress of 3.5 psi and
           i = 1.0.
           5
             Seam strength applies to both field and manufactured seams, if geotextile is
           seamed.
                   Table 5-1. Recommended Geotextile Property Requirements for Railroad Applications.
subgrade bearing pressure. In the design of new                5-5. Protective Sand layer
track construction or track rehabilitation using                 a. Although not normally required, a 2-inch-
geotextiles, the geotextile should be placed at the            thick layer placed over the geotextile may assist in
deeper of the following:                                       reducing the abrasion forces caused by the ballast
  a. At least 12 inches below the cross tie.                   as well as provide an additional filtration layer. In
                                                               track rehabilitation where undercutting or plow-
  b. At the bottom of the ballast layer in the case            ing type of ballast removal operation is used, there
of rehabilitation by plowing.                                  may be many large aggregate pieces remaining on
  c. At the bottom of the subballast in new con-               the surface of the subgrade prior to the placement
struction or rehabilitation where the track is                 of the geotextile. A 2-inch-thick layer of sand
removed.                                                       placed on the subgrade provides a smooth surface

5-2
                                                                                           TM 5-818-8/AFJMAN 32-1030

for the placement of the geotextile and protects the                  placed in the trenches to facilitate movement of
geotextile from punctures and abrasion due to the                     the water from the track.
large aggregate pieces that are on the subgrade.                         b. Highway Grade Crossings.
   b. While the use of protective clean sand (less                          (1) Drainage in a grade crossing is generally
than 5 percent passing the No. 200 sieve) extends                     parallel to the rails until the pavement and road
service life of a geotextile, there are also several                  shoulder have been cleared. Once clear of the
disadvantages. These disadvantages include the                        crossing itself, the drainage should be turned
extra cost of the sand, the increase in rail height                   perpendicular to the track and discharged away
(which results from the extra thickness in the                        from the track structure. A perforated drain pipe,
track structure), and the difficulty and cost of                      either wrapped with a geotextile during installa-
placing the sand layer during construction or                         tion or prewrapped, may be placed in the trench to
rehabilitation.                                                       assist the flow of water from within the crossing to
                                                                      the ditches outside of the crossing area. Such
5-6. Drainage                                                         drainpipes should be placed in the trench with the
Adequate drainage is the key to a stable railroad                     line of perforations facing downward. The ends of
track structure. During the design of a new track                     the perforated drainpipes and the geotextile under
or a track rehabilitation project, provisions for                     the crossing should be laid with sufficient fall
improving both internal and external track drain-                     toward the side ditches to prevent water from
age should be included. Drainage provisions that                      ponding in the crossing area. Whether perforated
should be considered include adequate (deep) side                     pipes are used or not, the shoulders at the corner
ditches to handle surface runoff, sufficient crown                    of the crossing should be removed, and the ends of
in both the subgrade and subballast layers to                         the geotextile turned down so that the geotextile
prevent water from ponding on the top of the                          facilitates drainage under gravity toward the side
subballast or subgrade, installation of perpendicu-                   ditches.
lar drains to prevent water accumulation in the                             (2) In cold climates it is common to salt and
track, and French drains where required to assist                     sand highways, including grade crossings, which
in the removal of water from the track structure.                     can lead to ballast fouling in the grade crossing.
During track rehabilitation, the creation of bath-                    One method of preventing or minimizing this
tub or canal effects should be avoided by having                      ballast fouling is to encapsulate the ballast in a
the shoulders of the track below the level of the                     geotextile. The provision for drainage in this type
ballast/geotextile/subgrade interface. Geotextiles                    of installation would be the same as discussed
should not be placed in a railroad track structure                     above.
until existing drainage problems are corrected.                           c. Turnout Applications.
Proper maintenance of railroad drainage facilities                          (1) The installation of a geotextile under a
is described in TM 5-627.                                             turnout is basically the same as installation in
                                                                       any other segment of track. In the vicinity of a
5-7. Typical Sections                                                  switch, drainage of ballast or subballast to ditches
Figure 5-1 presents typical cross sections of the                      is more difficult to achieve because horizontal
railroad track structure showing the recommended                       distances for subsurface flow are about doubled
use of a geotextile in the track.                                      and gradients are about halved. Thus, there are
                                                                       reasons for using geotextiles to promote lateral
5-8. Special Applications                                              drainage under a turnout where none is used in
   a. Installation of G e o t e x t i l e s B e l o w N a t u r a l    adjacent straight sections. If this is done, it should
Ground Level. In some locations, the elevation of
                                                                       extend at least 25 feet away from the turnout
the track structure may be such that the geotex-
                                                                       itself to provide a transition section. As with road
tile is placed below the level of the natural
                                                                       crossings, particular attention should be given to
ground. Where the natural ground surface is ele-
                                                                       the removal of surface water from the turnout
vated above the geotextile, steps should be taken
                                                                       area.
to prevent the inflow of water. A French drain                               (2) Many geotextile manufacturers produce
installed along the edge of the track and lined or                     specially packaged units ready-made for quick
completely encapsulated in a geotextile to filter                      application under turnouts varying from No. 8 to
the inflow of surface water may be used to direct                      No. 20.
water away from the track structure. In extremely                         d. Rail Crossings (Diamonds). The use of a
flat areas it may be necessary to construct perpen-                    geotextile in the track under a rail crossing is very
dicular side ditches and soak-away pits from the                       similar to the road crossing application. The de-
track structure to allow the water to drain out of                     sign and installation process must provide ade-
the French drains. Slotted drain pipes can be                          quate drainage.
                                                                                                                         5-3
TM 5-818-8/AFJMAN 32-1030




                            Figure 5-1. Typical sections of Railroad Track with Geotextile




5-4
                                                                              TM 5-818-8/AFJMAN 32-1030

                                                CHAPTER 6

                               EROSION AND SEDIMENT CONTROL


6-1. Erosion Control                                     Table 6-1 gives recommended minimum strength
                                                         values.
Erosion is caused by a group of physical and
                                                               (3) Cover material. The cover material (gravel,
chemical processes by which the soil or rock
                                                         rock fragments, riprap, armor stone, concrete
material is loosened, detached, and transported
                                                         blocks, etc.) is a protective covering over the
from one place to another by running water,
                                                         geotextile that minimizes or dissipates the hydrau-
waves, wind, moving ice, or other geological sheet
                                                         lic forces, protects the geotextile from extended
and bank erosion agents. Clayey soils are less
                                                         exposure to UV radiation, and keeps it in intimate
erodible than fine sands and silts. See figure 6-1.
                                                         contact with the soil. The type, size, and weight of
This chapter covers the use of geotextiles to
                                                         cover material placed over the geotextile depends
minimize erosion caused by water.
                                                         on the kinetic energy of water. Cover material
6-2. Bank Erosion
                                                         that is lightweight in comparison with the hydrau-
                                                         lic forces acting on it may be moved. By removing
Riprap is used as a liner for ditches and channels       the weight holding the geotextile down, the
subjected to high-velocity flow and for lake, reser-     ground-water pressure may be able to separate the
voir and channel banks subject to wave action.           geotextile from the soil. When no longer con-
Geotextiles are an effective and economical alter-       strained, the soil erodes. The cover material must
native to conventional graded filters under stone        be at least as permeable as the geotextile. If the
riprap. However, for aesthetic or economic reasons,      cover material is not permeable enough, a layer of
articulated concrete mattresses, gabions, and pre-       fine aggregate (sand, gravel, or crushed stone)
cast cellular blocks have also been used to cover        should be placed between it and the geotextile. An
the geotextile. The velocity of the current, the         important consideration in designing cover mate-
height and frequency of waves and the erodibility        rial is to keep the void area between stones
of the bank determine whether bank protection is         relatively small. If the void area is excessively
needed. The geotextiles used in bank protection          large, soils may move from areas weighted by
serve as a filter. Filter design is covered in chapter   stones to unweighted void areas between the
3.                                                       stones, causing the geotextile to balloon or eventu-
   a. Special Design Considerations.                     ally rupture. The solution in this case is to place a
     (1) Durability. The term includes chemical,         graded layer of smaller stones below the large
biological, thermal, and ultraviolet (UV) stability.     stones that will prevent the soil from moving. A
Streams and runoff may contain materials that            layer of aggregate may also be needed if a major
can be harmful to the geotextile. When protected         part of the geotextile is covered as for example by
from prolonged exposure to UV light, the common          concrete blocks. The layer will act as a pore water
synthetic polymers do not deteriorate or rot in          dissipator.
prolonged contact with moisture. All geotextile                 (4) Anchorage. At the toe of the streambank,
specifications must include a provision for covering     the geotextile and cover material should be placed
the geotextile to limit its UV radiation exposure to      along the bank to an elevation below mean low
30 days or less.                                         water level to minimize erosion at the toe. Place-
     (2) Strength and abrasion resistance. The re-        ment to a vertical distance of 3 feet below mean
quired properties will depend on the specific appli-      low water level, or to the bottom of the streambed
cation-the type of the cover material to be used         for streams shallower than 3 feet, is recommended.
(riprap, sand bags, concrete blocks, etc.), the size,     At the top of the bank, the geotextile and cover
weight, and shape of the armor stone, the han-            material should either be placed along the top of
dling placement techniques (drop height), and the         the bank or with 2 feet vertical freeboard above
severity of the conditions (stream velocity, wave         expected maximum water stage. If strong water
height, rapid changes of water level, etc.). Abra-        movements are expected, the geotextile needs to be
sion can result from movement of the cover mate-          anchored at the crest and toe of the streambank
rial as a result of wave action or currents.              (fig 6-2).
Strength properties generally considered of pri-                (5) If the geotextile must be placed below low
mary importance are tensile strength, dimensional         water, a material of a density greater than that of
stability, tearing, puncture, and burst resistance.       water should be selected.
                                                                                                          6-l
 TM 5-818-8/AFJMAN 32-1030

 Table 6-1. Recommended Geotextile Mininmum Strength Re-              in case of UV protected and low UV susceptible
                       quirements.
                                                                      polymer geotextiles. The geotextile should be
 Type Strength Test Method                Class A
                                                    1
                                                        Class B
                                                                  2
                                                                      loosely laid, free of tension, folds, and wrinkles.
                                                                      When used for streambank protection, where cur-
 Grab Tensile     ASTM D   4632              200           90
 Elongation (%)   ASTM D   4632               15           15         rents acting parallel to the bank are the principal
 Puncture         ASTM D   4833               80           40         erosion forces, the geotextile should be placed with
 Tear             ASTM D   4533               50           30         the longer dimension (machine direction) in the
 Abrasion         ASTM D   3884               55           25         direction of anticipated water flow. The upper
 Seam             ASTM D   4632              180           80
 Burst            ASTM D   3786              320          140
                                                                      strips of the geotextile should overlap the lower
 1
                                                                      strips (fig 6-3). When used for wave attack or cut
    Fabrics are used under conditions more severe than Class B        and fill slope protection, the geotextile should be
 such as drop height less than 3 feet and stone weights should
                                                                      placed vertically down the slope (fig 6-3), and the
 not exceed 250 pounds.
 ‘
   Fabric is protected by a sand cushion or by zero drop height.      upslope strips should cover the downslope strips.
    b. Construction Considerations.                                   Stagger the overlaps at the ends of the strips at
      (1) Site preparation. The surface should be                     least 5 feet. The geotextile should be anchored at
 cleared of vegetation, large stones, limbs, stumps,                  its terminal ends to prevent uplift or undermining.
 trees, brush, roots, and other debris and then                       For this purpose, key trenches and aprons are used
 graded to a relatively smooth plane free of obstruc-                 at the crest and toe of the slope.
 tions, depressions, and soft pockets of materials.                        (3) Overlaps, seams, securing pins. Adjacent
      (2) Placement of geotextiles. The geotextile is                 geotextile strips should have a minimum overlap
 unrolled directly on the smoothly graded soil                        of 12 inches along the edges and at the end of
 surface. It should not be left exposed to UV                         rolls. For underwater placement, minimum over-
 deterioration for more than 1 week in case of                        lap should be 3 feet. Specific applications may
 untreated geotextiles, and for more than 30 days                     require additional overlaps. Sewing, stapling, heat




                           Figure 6-1. Relationship between Atterberg Limits and Expected Erosion Potential.

6-2
                                                                                              TM 5-818-8/AFJMAN 32-1030

Table 6-2. Pin Spacing Requirements in Erosion Control Appli-           (4) Placement of cover material on geotextile.
                          cations.                                  For sloped surfaces, placement of the cover stone
Slope                                    Pin Spacing                or riprap should start from the base of the slope
                                             feet                   moving upward and preferably from the center
                                               2
                                                                    outward to limit any partial movement of soil
Steeper than 1V on 3H
1V on 3H to 1V on 4H                           3                    because of sliding. In no case should drop heights
Flatter than 1V on 4H                          5                    which damage the geotextile be permitted. Testing
                                                                    may be necessary to establish an acceptable drop
V = vertical; H = horizontal.
                                                                    height.
welding, or gluing adjacent panels, either in the
factory or on site, are preferred to lapping only.                   6-3. Precipitation Runoff Collection and Diver-
Sewing has proved to be the most reliable method                     sion Ditches
of joining adjacent panels. It should be performed                   A diversion ditch is an open, artificial, gravity
using polyester, polypropylene, kevlar or nylon                      flow channel which intercepts and collects precipi-
thread. The seam strength for both factory and                       tation runoff, diverts it away from vulnerable
field seams should not be less than 90 percent of                    areas, and directs it toward stabilized outlets. A
the required tensile strength of the unaged geotex-                  geotextile or revegetation mat can be used to line
tile in any principal direction. Geotextiles may be                  the ditch. It will retard erosion in the ditch, while
held in place on the slope with securing pins prior                  allowing grass or other protective vegetation
to placing the cover material. These pins with                       growth to take place. The mat or geotextile can
washers should be inserted through both strips of                    serve as additional root anchoring for some time
the overlapped geotextile along a line through the                   after plant cover has established itself if UV
midpoint of the overlap. The pin spacing, both                       resistant geotextiles are specified. Some materials
along the overlaps or seams, depends on the slope,                   used for this purpose are designed to degrade after
as specified in table 6-2. Steel securing pins, 3/16                 grass growth takes place. The geotextile can be
inch in diameter, 18 inches long, pointed at one                     selected and specified using physical properties
end, and fitted with a l.5-inch metal washer on                      indicated in table 6-1 and the filter criteria of
the other have performed well in rather firm soils.                  chapter 3. Figure 6-4 shows a typical example.
Longer pins are advisable for use in loose soils.
The maximum slope on which geotextiles may be                        6-4. Miscellaneous Erosion Control
placed will be determined by the friction angles                     Figures 6-5 and 6-6 show examples of geotextile
between the natural-ground and geotextile and                        applications in erosion control at drop inlets and
 cover- material and geotextile. The maximum al-                     culvert outlets and scour protection around
 lowable slope in no case can be greater than the                    bridges, piers, and abutments. Design criteria sim-
 lowest friction angle between these two materials                   ilar to that used for bank protection should be
 and the geotextile.                                                 used for these applications.




                                Figure 6-2. Pin Spacing Requirements in Erosion Control Applications.

                                                                                                                      6-3
TM 5-818-8/AFJMAN 32-1030




             Figure 6-3. Geotextile Placement for Currents Acting Parallel to Bank or for Wave Attack on the Bank.
6-5. Sediment Control                                             support component may be a wire or plastic mesh
Silt fences and silt curtains are sediment control                support fence attached to support posts or in some
systems using geotextiles.                                        cases may be support posts only. The designer has
   a. Silt Fence. A silt fence is a temporary vertical            to determine the minimum height of silt fences,
barrier composed of a sheet of geotextile supported               and consider the geotextile properties (tensile
by fencing or simply by posts, as illustrated in                  strength, permeability) and external factors (the
figure 6-5. The lower end of the geotextile is                    slope of the surface, the volume of water and
buried in a trench cut into the ground so that                    suspended particles which are delivered to the silt
runoff will not flow beneath the fence. The purpose               fence, and the size distribution of the suspended
of the permeable geotextile silt fence is to inter-               particles). Referring to figure 6-7, the total height
cept and detain sediment from unprotected areas                   of the silt fence must be greater than
before it leaves the construction site. Silt fence are            ; where is the height of geotextile necessary to
sometimes located around the entire downslope                     allow water flowing into the basin to flow through
portion or perimeter of urban construction sites.                 the geotextile, considering the permeability of the
Short fences are often placed across small drainage               geotextile;      is the height of water necessary to
ditches (permanent or temporary) constructed on                   overcome the threshold gradient of the geotextile
the site. Both applications are intended to function              and to initiate flow. For most expected conditions,
for one or two construction seasons or until grass                           is about 6 inches or less. The silt fence
sod is established. The fence reduces water veloc-                accomplishes its purpose by creating a pond of
ity allowing the sediment to settle out of suspen-                relatively still water which serves as a sedimenta-
sion.                                                             tion basin and collects the suspended solids from
     (1) Design concepts. A silt fence consists of a              the runoff. The useful life of the silt fence is the
sheet of geotextile and a support component. The                  time required to fill the triangular area of height

6-4
                                                                                TM 5-818-8/AFJMAN 32-1030




                           CENTER LINE PROFILE OF GEOTEXTILE-LINED DITCH
                                             Figure 6-4. Ditch Liners.
h (fig 6-7) behind the silt fence with sediment. The        the sediment-filled water through the geotextile.
height of the silt fence geotextile should not                   (4) Required geotextile properties. The geotex-
exceed 3 feet.                                              tile used for silt fence must also have:
     (2) Design for maximum particle retention.                    (a) Reasonable puncture and tear resistance
Geotextiles selected for use in silt fences should          to prevent damage by floating debris and to limit
have an AOS that will satisfy the following equa-           tearing where attached to posts and fence.
tion with a limiting value equal to the No. 120                    (b) Adequate resistance to UV deterioration
sieve size.                                                 and biological, chemical, and thermal actions for
                                                            the desired life of the fence.
                                               (eq 6-l)          (5) Construction considerations.
                                                                   (a) Silt fences should be constructed after
A minimum of 90-pound tensile strength (ASTM D              the cutting of trees but before having any sod
4632 Grab Test Method) is recommended for use               disturbing construction activity in the drainage
with support posts spaced a maximum of 8 feet               area.
apart.                                                             (b) It is a good practice to construct the silt
     (3) Design for filtration efficiency. The geotex-      fence across a flat area in the form of a horseshoe.
tile should be capable of filtering most of the soil        This aids in the ponding of the runoff, and in-
particles carried in the runoff from a construction         creases the strength of the fence. Prefabricated silt
site without unduly impeding the flow. ASTM D               fence sections containing geotextile and support
5141 presents the laboratory test used to deter-            posts are commercially available. They are gener-
mine the filtering efficiency and the flow rate of          ally manufactured in heights of 18 and 36 inches.

                                                                                                              6-5
TM 5-818-8/AFJMAN 32-1030

At the lower portion of the silt fence, the geotex-           about 100 feet long and of any required width. An
tile is extended for burying anchorage.                       end connector is provided at each end of the
   b. Silt Curtains. A silt curtain is a floating             section for fastening sections together. Anchor
vertical barrier placed within a stream, lake, or             lines hold the curtain in a configuration that is
other body of water generally at runoff discharge             usually U-shaped, circular, or elliptical. The de-
points. It acts as a temporary dike to arrest and             sign criteria and properties required for silt fences
control turbidity. By interrupting the flow of wa-            also apply to silt curtains. Silt curtains should not
ter, it retains suspended particles; by reducing the          be used for:
velocity, it allows sedimentation. A silt curtain is               (1) Operations in open ocean.
composed of a sheet of geotextile maintained in a
                                                                   (2) Operations in currents exceeding 1 knot.
vertical position by flotation segments at the top
and a ballast chain along the bottom. A tension                    (3) Areas frequently exposed to high winds
cable is often built into the curtain immediately             and large breaking waves.
above or below the flotation segments to absorb                    (4) Around hopper or cutterhead dredges
stress imposed by currents and other hydrody-                 where frequent curtain movement would be neces-
namic forces. Silt curtain sections are usually               sary.




                            Figure 6-5. Use of Geotextiles near Small Hydraulic Structures.




                                          SCOUR PROTECTION FOR BRIDGE PIER

                              Figure 6-6. Use of Geotextiles around Piers and Abutments.

6-6
                                               TM   5-818-8/AFJMAN 32-1030




Figure 6-7. Sedimentation behind Silt Fence.




                                                                        6-7
                                                                            TM 5-818-8/AFJMAN 32-1030

                                               CHAPTER 7

                                      REINFORCED SOIL WALLS


7-1. Geotextile-Reinforced Soil Walls                     c. The construction of geotextile-reinforced walls
Soil, especially granular, is relatively strong under   in cut regions requires a wider excavation than
compressive stresses. When reinforced, significant      conventional retaining walls.
tensile stresses can be carried by the reinforce-         d. Excavation behind the geotextile-reinforced
ment, resulting in a composite structure which          wall is restricted.
possesses wider margins of strength. This extra
strength means that steeper slopes can be built.        7-4. Uses
Geotextiles have been utilized in the construction      Geotextile-reinforced walls can be substantially
of reinforced soil walls since the early 1970’s.        more economical to construct than conventional
Geotextile sheets are used to wrap compacted soil       walls. However, since geotextile application to
in layers producing a stable composite structure.       walls is relatively new, long term effects such as
Geotextile-reinforced soil walls somewhat resemble      creep, aging, and durability are not known based
the popular sandbag walls which have been used          on actual experience. Therefore, a short life, seri-
for some decades. However, geotextile- reinforced       ous consequences of failure, or high repair or
walls can be constructed to significant height          replacement costs could offset a lower first cost.
because of the geotextile’s higher strength and a       Serious consideration should be given before utili-
simple mechanized construction procedure.               zation in critical structures. Applications of
7-2. Advantages of Geotextile-Reinforced                geotextile-reinforced walls range from construction
Walls                                                   of temporary road embankments to permanent
                                                        structures remedying slide problems and widening,
Some advantages of geotextile-reinforced walls
                                                        highways effectively. Such walls can be con-
over conventional concrete walls are the following:
                                                        structed as noise barriers or even as abutments for
   a. They are economical.
                                                        secondary bridges. Because of their flexibility,
   b. Construction usually is easy and rapid. It
                                                        these walls can be constructed in areas where poor
does not require skilled labor or specialized equip-
                                                        foundation material exists or areas susceptible to
ment. Many of the components are prefabricated
                                                        earthquake activity.
allowing relatively quick construction.
   c. Regardless of the height or length of the wall,   7-5. General Considerations
support of the structure is not required during
                                                           a. The wall face may be vertical or inclined.
construction as for conventional retaining walls.
                                                        This can be because of structural reasons (internal
   d. They are relatively flexible and can tolerate
                                                        stability), ease of construction, or architectural
large lateral deformations and large differential
                                                        purposes. All geotextiles are equally spaced so that
vertical settlements. The flexibility of geotextile-
                                                        construction is simplified. All geotextile sheets,
reinforced walls allows the use of a lower factor of
                                                        except perhaps for the lowest one, usually extend
safety for bearing capacity design than for conven-
                                                        to the same vertical plane.
tional more rigid structures.
   e. They are potentially better suited for earth-        b. Geotextiles exposed to UV light may degrade
quake loading because of the flexibility and inher-     quite rapidly. At the end of construction, a protec-
ent energy absorption capacity of the coherent          tive coating should be applied to the exposed face
earth mass.                                             of the wall. An application of 0.25 gallon per
                                                        square yard of CSS-1 emulsified asphalt or spray-
7-3. Disadvantages of Geotextile-Reinforced             ing with a low viscosity water-cement mixture is
Walls                                                   recommended. This cement mixture bonds well
Some disadvantages of geotextile-reinforced walls       and provides satisfactory protection even for
over conventional concrete walls are the following:     smooth geotextiles. To protect the face of the wall
   a. Some decrease in geotextile strength may          from vandalism, a 3-inch layer of gunnite can be
occur because of possible damage during construc-       applied. This can be done by projecting concrete
tion.                                                   over a reinforcing mesh manufactured from No. 12
   b. Some decrease in geotextile strength may          wires, spaced 2 inches in each direction, supported
occur with time at constant load and soil tempera-      by No. 3 rebars inserted between geotextile layers
ture.                                                   to a depth of 3 feet.

                                                                                                        7-l
TM 5-818-8/AFJMAN 32-1030

  c. When aesthetic appearance is important, a                   centimeters per second. The ranking order indi-
low-cost solution like the facing system comprised               cates that gravels are not at the top. Although
of used railroad ties or other such materials can be             they posses high permeability and, possibly, high
used.                                                            strength, their utilization requires special atten-
   d. No weepholes are specified, although after                 tion. Gravel, especially if it contains angular
UV and vandal protection measures the wall face                  grains, can puncture the geotextile sheets during
may be rather impermeable. To ensure the fast                    construction. Consequently, consideration must be
removal of seeping water in a permanent struc-                   given to geotextile selection so as to resist possible
ture, it is recommended to replace 1 to 2 feet of                damage. If a geotextile possessing high puncture
the natural foundation soil (in case it is not                   resistance is available, then GP and GW should
free-draining) with a crushed-stone foundation                   replace SP and SW, respectively, in their ranking
layer to facilitate drainage from within and behind              order. The retained soil unit weight should be
the wall. The crushed rock may be separated from                 specified based on conventional laboratory compac-
the natural soil by a heavy weight geotextile                    tion tests. A minimum of 95 percent of the maxi-
meeting filter criteria of chapter 3.                            mum dry unit weight, as determined by ASTM D
                                                                 698 should be attained during construction. Since
7-6. Properties of Materials
                                                                 the retained soil will probably be further densified
  a. Retained Soil. The soil wrapped by the geo-                 as additional layers are placed and compacted, and
textile sheets is termed “retained soil.” This soil              may be subjected to transitional external sources
must be free-draining and nonplastic. The ranking                of water, such as rainfall, it is recommended for
(most desirable to less desirable) of various re-                design purposes that the saturated unite weight be
tained soils for permanent walls using the Unified               used.
Soil Classification System is as follows: SW, SP,                   b. Backfill Soil. The soil supported by the rein-
GW, GP, and any of these as a borderline classifi-               forced wall (the soil to the right of L in figure 7-1)
cation which is dual designated with GM or SM.                   is termed “backfill soil.” This soil has a direct
The amount of fines in the soil is limited to 12                 effect on the external stability of the wall. There-
percent passing sieve No. 200. This restriction is
                                                                 fore, it should be carefully selected. Generally,
imposed because of possible migration of fines                   backfill specifications used for conventional retain-
being washed by seeping water. The fines may be                  ing walls should be employed here as well. Clay,
trapped by geotextile sheets, thus eventually creat-             silt, or any other material with low permeability
ing low permeability liners. Generally, the perme-               should be avoided next to a permanent wall. If low
ability of the retained soil must be more than 10-3              quality materials are used, then a geotextile filter




             Figure 7-1. General Configuration of a Geotextile Retained Soil Wall and Typical Pressure Diagrams.



7-2
                                                                             TM 5-818-8/AFJMAN 32-1030

meeting filtration requirements of chapter 3               z = vertical distance from load to point where
should be placed to separate the fines from the                stress is being calculated
free draining backfills, thus preventing fouling of
the higher quality material. Since the retained soil       y = horizontal distance from load to wall, and
and backfill may have an effect on the external                parallel to the wall
stability of the reinforced wall, the properties of     A typical live load pressure distribution is shown
both materials are needed. The unit weight should       in figure 7-1b. Figure 7-2 illustrates live load
be estimated as for the retained soil; use the          stress calculations.
maximum density at zero air voids. The strength            c. Fabric Tension. Tension in any fabric layer is
parameters should be determined using drained           equal to the lateral stress at the depth of the layer
direct shear tests (ASTM D 3080) for the perme-         times the face area that the fabric must support.
able backfill. The backfill and the retained soil       For a vertical fabric spacing of X , a unit width of
must have similar gradation at their interface so       fabric at depth d must support a force of          ,
as to minimize the potential for lateral migration      where       is the average total lateral pressure
of soil particles. If such requirement is not practi-   (composite of dead plus live load) over the vertical
cal, then a conventional soil filter should be          interval X .
designed, or a geotextile filter used along the            d. Pullout Resistance. A sufficient length of
interface.                                              geotextile must be embedded behind the failure
7-7. Design Method                                      plane to resist pullout. Thus, in Figure 7-1a, only
                                                        the length, Le, of fabric behind the failure plan
The design method recommended for retaining             AB would be used to resist pullout. Pullout resis-
walls reinforced with geotextiles is basically the      tance can be calculated from:
U.S. Forest Service method as developed by Stew-
ard, Williamson, and Mahoney (1977) using the                                                       (eq 7-4)
Rankine approach. The method considers the earth        where
pressure, line load pressure, fabric tension, and            = pullout resistance
pullout resistance as the primary design parame-           d = depth of retained soil below top of retain-
ters.                                                           ing wall
   a. Earth Pressure. Lateral earth pressure at any           = unit weight of retained soil
depth below the top of the wall (fig 7-1a) is given           = angle of internal friction of retained soil
by:                                                           = length of embedment behind the failure
                                            (eq 7-1)             plane

where                                                   It can be seen from this expression that pullout
      = lateral earth pressure acting on the wall       resistance is the product of overburden pressure,
      = at rest pressure coefficient                       , and the coefficient of friction between retained
     = soil unit weight                                 soil and fabric which is assumed to be TAN
   d = depth below the top of the wall                  This resistance is in pounds per square foot which
                                                        is multiplied by the surface area of       for a unit
A typical earth pressure distribution is shown in
                                                        width. Where different soils are used above and
figure 7-1b. Use of the “at rest” pressure coeffi-      below the fabric layer, the expression is modified
cient, Ko , is recommended and is determined by         to account for different coefficients of friction for
the following equation:
                                                        each soil:
                                            (eq 7-2)                                                (eq 7-5)
where is the angle of internal friction of the soil.
The failure surface, AB in figure 7-1a, slopes          7-8.Design Procedure
upward at an angle of                                   The recommended design procedure is discussed in
   b. Live Load Pressure. Lateral pressures from        the following steps. The calculations for the fabric
live loads are calculated for a point load acting on    dimensions for overlap, embedment length and
the surface of the backfill using the following         vertical spacing should include a safety factor of
equation:                                               1.5 to 1.75 depending upon the confidence level in
                                         (eq 7-3)       the strength parameters.
                                                          a. Retained Soil Properties   and . Only free-
where                                                   draining granular materials should be used as
   P = vertical load
                                                        retained soil. The friction angle,        , will be
   x = horizontal distance from load to wall and        determined using the direct shear (ASTM D 3080)
       perpendicular to the wall

                                                                                                         7-3
TM 5-818-8/AFJMAN 32-1030




                                          B. METHOD OF REPRESENTING TRAFFIC LIVE LOADS


             Figure 7-2.   Procedures for Computing Live Load Stresses on Geotextile Reinforced Retaining Walls.
or triaxial tests (ASTM D 2850). The unit weight,                 three locations along the wall are checked to
   , will be determined in a moisture density test                determine the most critical.
(ASTM D 698). Generally, 95 percent of ASTM D                       d. Composite Pressure Diagram. The earth pres-
698 maximum density can be easily attained with                   sure and live load pressure diagrams are combined
granular materials. However, other densities can                  to develop the composite diagram used for design
be specified so long as the friction angle used is                as shown in Figure 7-1b.
consistent with that density. The saturated unit                    e. Vertical Spacing of the Fabric Layer. To deter-
weight is used in lateral pressure calculations.                  mine the vertical strength of the fabric layer, the
  b. Lateral Earth Pressure Diagram. Using the                    fabric allowable tensile strength, S , is set equal to
properties of the retained soil, calculate the pres-              the lateral force calculated from        , where    is
sure coefficient,                The lateral earth                the lateral pressure at the middle of the layer.
pressure expression:                                              Thus, knowing the fabric tensile strength, and
                                                                  value of      the fabric vertical spacing, X , can be
                                                                  calculated. The fabric strength should be divided
is used to calculate the triangular shaped pressure               by the appropriate safety factor. The equation for
distribution curve for the height of retaining wall
                                                                  fabric spacing is:
desired.
   c. Live Load Lateral Pressure Diagram. It is
first necessary to determine the design load. Lat-                                                                 (eq 7-6)
eral pressure diagrams must be developed for each
vehicle or other equipment expected to apply loads                  f. Length of Fabric Required to Develop Pullout
to the retaining wall using equation 7-3. The                     Resistance. The formula for pullout resistance,
equation is solved for each wheel and the results                                   , is used to solve for the pullout
added to obtain the lateral pressure. This pressure               resistance which can be developed at a given depth
is calculated at 2-foot vertical intervals over the               geotextile length combination or to solve for d ,
height of the retaining wall. Normally, from one to               the depth required to develop      . The usual case
                                                                  for walls is to set        equal to the geotextile
7-4
                                                                           TM 5-818-8/AFJMAN 32-1030

strength and solve for    , the length of geotextile   This can be solved for the length of overlap,
required. Thus, the expression would be:               required:

                                            (eq 7-7)                                            (eq 7-11)

where                                                  The minimum length of overlap should be 3 feet to
      = fabric tensile strength                        ensure adequate contact between layers.
   F.S. = safety factor of 1.5 to 1.75                    h. External Wall Stability. Once the internal
The minimum length of the fabric required is 3         stability of the structure is satisfied, the external
feet.                                                  stability against overturning, sliding and founda-
  g. Length of Fabric Overlap for the Folded           tion bearing capacity should be checked. This is
Portion of Fabric at the Face. The overlap,       ,    accomplished in the same manner as for a retain-
must be long enough to transfer the stress from        ing wall without a geotextile. Overturning loads
the lower section of geotextile to the longer layer    are developed from the lateral pressure diagram
above. The pullout resistance of the geotextile is     for the back of the wall. This may be different
given by:                                              from the lateral pressure diagram used in check-
                                                       ing internal stability, particularly due to place-
                                            (eq 7-8)   ment of live loads. Overturning is checked by
where        = depth to overlap. Tension in the        summing moments of external forces about the
geotextile is:                                         bottom at the face of the wall. Sliding along the
                                                       base is checked by summing external horizontal
                                            (eq 7-9)   forces. Bearing capacity is checked using standard
                                                       foundation bearing capacity analysis. Theoreti-
Since the factor of safety can be expressed as:        cally, the fabric layers at the base could be shorter
                                                       than at the top. However, because of external
                                           (eq 7-10)   stability considerations, particularly sliding and
                                                       bearing capacity, all fabric layers are normally of
                                                       uniform width.




                                                                                                     7-5
                                                                          TM 5-818-8/AFJMAN 32-1030

                                           APPENDIX A

                                            REFERENCES


Government Publications
   Departments of the Army and the Air Force
   TM 5-820-2/AFJMAN 32-1016                 Drainage and Erosion Control Subsurface Drainage Facil-
                                               ities for Airfield Pavements
   TM 5-822-5/AFJMAN 32-1018                 Pavement Design for Roads, Streets, Walks, and Open
                                               Storage Areas
   TM 5-850-2/AFJMAN 32-1014                 Flexible Pavement Design for Airfields
   TM 5-825-3/AFJMAN 32-1014                 Rigid Pavements for Airfields
   TM 5-850-2/AFJMAN 32-1046                 Railroad Design and Construction at Army and Air Force
                                               Installations
   TM 5-627                                  Maintenance of Trackage
   Federal Highway Administration, 400 Seventh Street, SW, Washington, DC
   FHWA-HI-90-001                            Geotextile Design and Construction Guidelines October,
                                               1989
   United States Department of Agriculture Forest Service, Portland, Oregon
   Guidelines for Use of Fabrics in Construction and Maintenance of Low Volume Roads (June 1977)
Nongovernment Publications
   American Society for Testing and Materials (ASTM), 1916 Race St., Philadelphia, PA 19103
   D 276-93                                 Identification of Fibers in Textiles
   D 698-91                                 Laboratory Compaction Characteristics of Soil Using Stan-
                                               dard Effort
   D 1557-91                                Laboratory Compaction Characteristics of Soil Using Modi-
                                              fied Effort
   D 1683-90                                Failure in Sewn Seams of Woven Fabrics
   D 2850-87                                Unconsolidated, Undrained Strength of Cohesive Soils in
                                               Triaxial Compression
   D 3080-90                                Direct Shear Test of Soils Under Consolidated Drained
                                               Conditions
   D 3776-85 (1990)                         Mass per Unit Area (Weight) of Woven Fabric
   D 3786-87                                Hydraulic Bursting Strength of Knitted Goods and Non-
                                               woven Fabrics-Diaphragm Bursting Strength Tester
                                               Method
   D 4355-92                                Deterioration of Geotextiles from Exposure to Ultraviolet
                                               Light and Water (Xenon-Arc Type Apparatus)
   D 4491-92                                Water Permeability of Geotextiles by Permittivity
   D 4533-91                                Trapezoid Tearing Strength of Geotextiles
   D 4595-93                                Tensile Properties of Geotextiles by the Wide Strip Method
   D 4632-91                                Breaking Load and Elongation of Geotextiles (Grab Method)
   D 4716-87                                Constant Head Hydraulic Transmissivity (In-Plane Flow) of
                                               Geotextiles and Geotextile Related Products
   D 4751-87                                Determining the Apparent Opening Size of a Geotextile
   D 4833-88                                Index Puncture Resistance of Geotextiles, Geomembranes,
                                               and Related Products
   D 5101-90                                Measuring the Soil Geotextile System Clogging Potential by
                                               the Gradient Ratio
   D 5141-91                                Determining Filtering Efficiency and Flow Rate of a Geo-
                                               textile for Silt Fence Application Using Site Specific Soil




                                                                                                    A-1
TM 5-818-8/AFJMAN 32-1030

   American Association of State Highway and Transportation Officials, 444 N. Capitol Street, N.W.,
   Suite 225, Washington, DC 20001
   M 288-90                              Standard Specification for Geotextiles, Asphalt Retention,
                                           and Area Change of Paving Engineering Fabrics




A-2
                                           BIBLIOGRAPHY


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    Conference, W. Lebanon, NH, February 26-28, 1991, p. 178-190.
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De Ment, L. E., “Two New Methods of Erosion Protection for Louisiana,” Shore Beach, Vol. 45, No. 1, p. 8,
    1977.
Douglas, G. E., “Design and Construction of Fabric-Reinforced Retaining Walls by New York State,”
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El-Fermaoui, A., and Nowatzki, E., “Effect of Confining Pressure on Performance of Geotextiles in Soils,”
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Ford, H. W., “Estimating the Potential for Ochre Clogging Before Installing Drains,” Transactions of the
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Ford, H. W., “Some Fundamentals of Iron and Slime Deposition in Drains,” Proceedings of the Second
    International Drainage Workshop, Washington, DC, pp. 207-212, 1982b.
Fowler, Jack, “Analysis of Fabric-Reinforced Embankment Test Section at Pinto Pass, Mobile, Alabama,”
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Fowler, J., “Theoretical Design Considerations for Fabric-Reinforced Embankments,” Proceedings of the
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Giroud, J. P., “Filter Criteria for Geotextiles,” Proceedings of the Second International Conference on
    Geotextiles, Vol. I, pp. 103-108, 1982.
Gulden, W., and Brown, D., “Treatments for Reduction of Reflective Cracking of Asphalt Overlays on
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Haliburton, T. A., “Design of Test Section for Pinto Pass Dike, Mobile, Alabama,” Report prepared by
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Haliburton, T. A., “Evaluation of Construction Procedure for Fabric-Reinforced Embankment Test Section,
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    Contract No. DACW39-78-M-4002, for US Army Engineer Waterways Experiment Station, Vicks-
    burg, Miss., 1979.
Haliburton, T. A., Anglin, C. C., and Lawmaster, J. D., “Selection of Geotechnical Fabrics for
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    Okla., 1978.
Haliburton, T. A., Fowler, J., and Langan, J. P., “Design and Construction of Fabric-Reinforced
    Embankment Test Section at Pinto Pass, Mobile, Alabama,” Transportation Research Record, 249, pp.
    27-34, Washington, DC, 1980.
Haliburton, T. A., Lawmaster, J. D., and King, J. J., “Potential Use of Geotechnical Fabric in Airfield
    Runway Design,” Contract No. AFOSR79-00871, Air Force Office of Scientific Research, School of
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Haliburton, T. A., Lawmaster, J. D., and McGuffie, V. C., “Use of Engineering Fabrics in Transportation
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    DTFH-80-C-0094, Stillwater, Okla., 1981.
Hammer, D. P., and Blackburn, E. D., “Design and Construction of Retaining Dikes for Containment of
    Dredged Material,” Technical Report TR-D-77-9, US Army Engineer District, Savannah, Savannah,
    Ga., 1977.
Henry, K. S., “Geotextiles as Capillary Barriers,” Geotechnical Fabrics Report, March/April, pp. 30-36.
Henry, K. S., “Laboratory Investigation of the Use of Geotextiles to Mitigate Frost Heave,” CRREL Report
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Henry, K. S., “Use of Geotextiles to Mitigate Frost Heave in Soils,‘, in Proceedings, V International
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Henry, K. S., S. Taylor and J. Ingersoll, “Effects of Freezing on the Microstructure of Two Geotextiles,” in
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Henry, Karen S., “Effect of Geotextiles on Water Migration in Freezing Soils and the Influence of Freezing
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    Report No. EPA/600/2-86/085, US Environmental Protection Agency, Cincinnati, Ohio, 1986.
Ingold, T. S., “An Analytical Study of Geotextile Reinforced Embankments,” Proceedings of the 2nd
    International Conference on Geotextiles, Las Vegas, Aug l-6, Vol. 3, pp. 683-688, 1982.
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Jewell, R. A., “A Limit Equilibrium Design Method for Reinforced Embankments of Soft Foundations,”
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Jones, C. J. F. P., Earth Reinforcement and Soil Structures, Butterworth and Co., Ltd., London, 1985.
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                                                             TM 5-818-8/AFJMAN 32-1030


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